Several changes and additions to grid_test.py

Lots of changes and additions go into this commit. The biggest notes are
the following,

1. The mapping routine now works (and fast!) for nearest neighbor
technique. For an analysis grid with 10^6 grid points and CSAPR
surveillance data, clock time is ~ 45 seconds.
2. Barnes and Cressman schemes still have to be implemented.
3. I have begun creating radius of influence (ROI) functions for use
with a Cressman scheme.
4. Pertinent debugging info is now printed at will by setting debug =
True.
5. The user is now able to get the nearest neighbor distances as a field
in the output grid object by setting map_dist = True.

More to come...