ENH: Grid object

Adding the ability to exclude fields during the creation
of a Grid object, as well as the ability to add a new field
with the 'add_field' attribute similar to the Radar object.
Furthermore, I am adding some extra axes and dimensions to check
for when creating a Grid object. These axes and dimensions are
compatible with older versions of MMCG files.
ENH: GridMapDisplay

Adding several new attributes and plotting enhancements, including
methods to create nicely formatted plot titles. The Grid graph
object now has similar attributes to the Radar graph object; I tried
to keep naming conventions as similar as possible between these two.
There are still some issues to address with this commit though,
including getting the projection to work properly for Basemap displays.

Also adding a function to the date utilities module which returns
the start time found in a Grid.
MOD: GridMapDisplay

Making the necessary changes such that the Basemap plots work as
expected. One of the changes required was to make sure the grid origin
latitude and longitude were not NumPy arrays (of size 1) but scalar
values. This was previously affecting the output of pyproj.Proj() when
using these as lat_0 and lon_0.
ENH: Grid quality control

Adding a field 'despeckling' method to the Grid object. This method can
be used to remove outliers from gridded fields. The heavy lifting of
this routine is done in Fortran. This routine defines a 3-D box filter,
counts the total number of points within this box as well as how many of
these points are 'noise', and if this ratio is too high, flags the grid
point as noise.
MOD: minor changes
ADD: f2py wrappers

Adding the f2py extension to the configuration in setup.py for
grid_qc.f90, as well as the pyf file required for this configuration.
Merge branch 'grid' into winds
MOD: wind retrieval

Minor changes made to solve_wind_field. The grid branch has been merged
into the winds branch, giving the winds branch access to some of the key
updates in the Grid object, like adding and despeckling fields. This
also allows me to produce a wind retrieval with solve_wind_field.