Add option to specify path to a grid file

[mogres]

Use Case

cellPACK currently has a boolean option to load_from_grid_file which loads the grid from a predetermined path. It would be good to have an option in the config to specify a path to a file to load the grid from. This could be related to #202.

Solution

Change the load_from_grid_file option to accept path arguments. If the path does not exist or is missing, fall back to the predetermined path. Create and save a new grid file if none of the previous files exist.

This enhancement is particularly useful while packing complex grids.