Bugfix: mapObj inverse conversions when coordinate systems change

This merge will fix the bug detailed in https://github.com/vtsuperdarn/davitpy/issues/60.

To test the new code, simply run `python plotUtils.py`.

Other notable changes:
1) Added code to test the `__call__` method.
2) Added warning to `__init__` so that the user is aware that datetime is being set to current time when it isn't specified.