Deal with missing data values. Fixes gh-3

Change the ingestion routine so it doesn't crash when there are missing values
(ie empty cells in the spreadsheet).

It was falling over when trying to save empty measurements, as they are converted
to floats and python can't do float(''). So add a try/exception and don't do
anything with that measurement if it fails.

I also had a test file that was missing some land_dist and theta_is values.
Add a test for these, and replace with fill value -999 if they are missing.
Merge branch 'ingest_missing_data' into master. Github #3

Merge in the changes we made to ingest.py so it deals with missing
data in the input files. The cases included are:
- Missing measurement value: measurement is ignored
- Missing land_dist_is or thetas_is: value set to -999

Conflicts:
	Mermaid2/ingest_data/ingest.py
Merge image ingestion branch into master

Merge in the image ingestion code, as well as unit tests/functional
tests for the new code, from branch add_images. Github #13

Also make some changes to the API to enable images to be filtered
by MeasurementType and Region.

Currently supported data files are AATSR, MERIS, VIIRS

Usage:
I = IngestImages('intput_dir', 'output_dir')
I.ingest_all OR I.ingest_image('/path/to/metafile.json')

The procedure is:

1. IngestImages object is instantiated with an input and output
   directory location

2. Method ingest_image() or ingest_all() is called. (ingest_image
   takes a JSON filename as an argument, and processes that image;
   ingest_all loops through all the JSON files in the input directory)

3. Image metadata is read from the JSON filename

4. Data arrays are read from the input file associated with current
   JSON file. If variable names are not specified, the code defaults
   to reflectance for AATSR and VIIRS, and radiance for MERIS.

5. The data within the requested region of interest are extracted,
   and regridded to a regularly spaced, un-rotated grid (which is
   necessary for GeoTiff)

6. The new gridded data arrays are written out to a GeoTiff, with the
   same file name as the original file (just new extension), and put
   in directory output_directory_from_step_1/SITE/INSTRUMENT/YEAR

7. An RGB image is created and saved as a jpg in the same directory,
   for quick checking/display on web/etc

8. A database entry is created for the image. In the case of AATSR
   images, two database entries are created - one each for nadir and
   forward directions.

9. The processed JSON and data files are moved to an "ingested" folder
   within the input directory
Initial changes to documentation, ready for using gh-pages

Remove some hardwired paths, and update author copyright

Also add a shell script to commit changes to documentation to the
gh-pages website. Make your changes on the master branch, then when
ready run the new script to commit them to the gh-pages branch.
nb script only commits, doesn't push up to github. Do that manually
so that you are sure it is ready.
Add some branding to the documentation website

Add the TOUCAN logo to the website, and change the colours
(picking out some from the logo) and fonts to give a more
distinctive look to the documentation.

Create a new sphinx theme "toucan_theme" to hold the changes.
Refactor to reflect name change to TOUCAN

Change all file names and references from "mermaid2" to "toucan"
(except some comments referring to "mermaid format) which should stay.
Create README.md
Add .gitignore file

Add a gitgnore file, and set it to ignore all .pyc files
since we don't need these in the repository
Tidy ingestion docstrings and update web docs

Tidy up the docstrings in the ingestion routines so that they
work properly with sphinx.

Add ingest_images, ingest_images_geo_tools and ingest_images_file_readers
to the documentation page.
Allow time filtering of images

Add time filtering to ImageResource so that date ranges
can be queried. Format to then use in query string is
?time__lte=YYYY-MM-DD
Allow multiple images per JSON file

Change to allow many image files to be described by
one metadata file. read_meta_file now returns a list
of dictionaries, which we loop through.

Format of json files is now:
[
{ image file 1 key:value pairs },
{ image file 2 key:value pairs },
{ etc }
]

This means that even if only one image file is
specified in the json file, it still needs to be
nested within the square brackets, so that the result
is still a list (with one element).
Add MERIS correction

The MERIS reflectance files had incorrect magnitudes.
Add the solar irradiance correction, following what
DIMITRI does. It reads in a solar irradiance model
from an external text file, applies a date
correction, and then use this solar model to correct
the MERIS reflectance values.

Add an aux_files directory where we can keep any
external files that are required for the ingestion
(such as the solar irradiance model that is needed
here).