FIX: bug fix and minor enhancements

Fixing a minor bug in the Grid object as to how it handles fields with
incorrect shapes. Also adding the capability for the user to exclude fields
during a read and an add_field method to the Grid object, both of these
similar to their counterparts in the Radar object.
STY: PEP8 and docstring fix on grid.py
Merge pull request #2 from jjhelmus/pr_166

STY: PEP8 and docstring fix of grid.py

These are the style changes Jonathan requested before merging this branch into Py-ART's master branch.
Merge pull request #166 from kirknorth/grid-fix

FIX: Grid object