Grid Type should be an enum

[oelbert]

The grid_type variable indicates what the model's grid geometry is, and is currently an int, in keeping with the Fortran code. The main distinction is if grid_type is 0, 1, or 2 that specifies a cubed-sphere geometry (only 0 is currently supported, the gnomonic equidistant cubed-sphere), and if grid_type is 3 or greater the model is run with an orthogonal doubly-periodic grid (typically set with grid_type=4). Pace doesn't need grid_type to be an integer, and for clarity we should switch to an enum that governs the grid type in the code first with gnomonic cubed sphere and orthogonal members.

[FlorianDeconinck]

Additionally, any functionality around the that grid_type (testing if it's <X or ==X) should be made into a well named method on the enum.

Bonus point for a good str, repr and docstring