ENH: multi-radar mapping without field data copy.

pyart.io.map_to_grid will now work when multiple radars are passed and
copy_field_data is False.
Initial refactoring of PyGrid object
DOC: fixed link to source code in conf.py
PyGrid object refactoring finished
DOC: PyGrid related documentation
Merge pull request #50 from jjhelmus/master

PyGrid refactor