simplify setting up uniform grid physical properties

[bandre-ucar]

The current implementation of uniform grid reads data from the netcdf files. This currently requires one data point per grid cell. This should be simplified to allow specifing a single value that is replicated over the entire domain. This will allow arbitrarily changing the grid for testing pure advection, diffusion, reaction tests to generate smoother curves that are easier to compare to analytical solutions.

[jinyun1tang]

@bandre-ucar, do you mean the forcing data for each grid?