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.

This pull request performs the interpolation and extrapolation in the antenna
 coordinate which allow for filtering of nonphysical edges.  Pull Request #178
does this interpolation in Cartesian space which does not offer this filtering.

Thanks to @deeplycloudy for pointing out this bug