Add run time options to save/load route handles to/from external file.

[DusanJovic-NOAA]

Description

Computing remapping route handles in fv3atm is computationally expensive, especially for large model and output grids. Since these are not changed from run to run in production, we can speed up the initialization phase by saving computed route handles once, and use them in subsequent runs, speeding up the initialization.

Solution

Add run time options to save/load route handles to/from external file.