Added tests to __main__ for testing coordinate conversion.
Fixed bug in __call__. Also added warning when user doesn't specify datetime argument.
Removed extra self. Also added more testing.
Merge pull request #61 from vtsuperdarn/bugfix_mapObj_inverse_conversions

Bugfix: mapObj inverse conversions when coordinate systems change
provided tests for multiple-step conversions work for me results match expected for all tests.