Refactor IngestImages class

- data is now an instance attribute, rather than being passed as an argument
- Define the inputdir and outdir attributes in __init__()
- New methods ingest_all() and ingest_image() replace ingest_images(), so user
  can choose between ingesting a single image (by passing a metadata file name)
  or ingesting all the images in the inputdir
Add scaling to HDF files

Read in the scale factor and offset values from HDF file variables, then
apply to data arrays before doing any other processing.

This is not required for the N1 file reader, as there we use the high level
get_band method, instead of get_dataset, and the scaling is done automatically
behind the scenes.
Add quickview jpg plot

Add a method to produce a jpg RGB plot of the region, for quick checking.
Reference wavelengths are specified for R,G, and B and then the sensor bands
that are closest to these are extracted for the plot.

Also add a unit test for the new method.