Stuff in `pydarn` converted to work with `coord_conv`, plus some bugfixes

This is the continuation of #77 and #78, with the changes to `pydarn/plotting/fan.py` added.  This combination of everything in `pydarn` was necessary to prevent breaking two of the modules whenever the other was changed.  

See the old pull requests above for more information on the changes to `radFov` and `mapOverlay`.  The changes to fan are to make it use `coord_conv` and to align it with changes in `radFov` and `mapOverlay`.  A test is added as well.