BUG: Treat radar coordinates as gate centers in plots

The range, azimuth, elevation coordinates in radar files typically specify
the center of the gate, when plotting Py-ART uses these coordinates to
interpolate and extrapolate the gates edges which are then used to plot
the data.

Prior to this the centers were used to plot which resulted in the plots
missing the last range gate and the last ray.
Merge pull request #179 from jjhelmus/pcolor_bug

BUG: Treat radar coordinates as gate centers in plots

Thanks to @deeplycloudy for pointing out this bug
MAINT: NEXRAD archive file now returned with float32 fields
MAINT: added replace_existing parameter to add_field method.
Merge pull request #181 from jjhelmus/master

MAINT: float32 NEXRAD fields and replace_existing parameter