Support for visualizing and creating non-uniform Grids

[jjhelmus]

Non-uniform grids can be stored in the Grid object but the plotting routines assume that these grids are uniform in layout. Support should be added to the various GridDisplay classes to visualize grids with non-uniform layouts.

One non-uniform Grids can be visualized, support for creating such grids should be added to one or more mapping routines.

See #158 for discussion and an early implementation of mapping from radar gates to a non-uniform grid.