MAINT: Renamed and placed RadarMapDisplay in own file.

* RadarDisplay map class renamed as RadarMapDisplay for consistency with
  GridMapDisplay
* Moved RadarMapDisplay class from the radar_display.py file to
  radarmapdisplay.py file which makes conditionally import simplier.
MAINT: renamed radar_display.py radardisplay.py
MAINT: refactor of RadarMapDisplay object

RadarMapDisplay parameters now more in line with a merging of RadarDisplay
and GridMapDisplay.
TST: unit tests for radarmapdisplay.py
MAINT: fixed typos in graph modules
DOC: documentation for RadarMapDisplay class
TST: skip radarmapdisplay test if basemap module not available
Merge pull request #1 from jjhelmus/pr_124

Refactor, unit test and documentation for RadarMapDisplay object