Fix the "limits" argument to MPL 2D grid viewer

Fix #421

The "limits" argument was not being used correctly in
Matplotlib2DGridViewer internals. The min, max axes limits were never
set. The fix now set the axes limits correctly.
Merge branch 'issue421-Matplotlib2DGridViewer_limits' of github.com:wd15/fipy into wd15-issue421-Matplotlib2DGridViewer_limits
Merge branch 'wd15-issue421-Matplotlib2DGridViewer_limits' into develop