Add an option to input layerwise parameters via files

[axch]

For two reasons:

• Completeness of the interface, as it it not currently possible to specify parameters for more than 10,000 layers via parameters.in without recompiling the MIM core.
• Convenience: long before 10,000 layers, it is likely the user will wish to generate the layerwise parameter vectors programmatically, and it would be polite if they did not need to programmatically modify parameters.in to input them.

The layerwise parameters in question are ah, hmean, and g_vec.

[axch]

I presume this is a rather low-priority issue, as MIM's target users are unlikely to want to run MIM with very many layers -- if the number of layers becomes comparable to the horizontal resolution, perhaps one is looking for a general circulation model instead. However, it seemed appropriate to mention.

Also, various rewrites currently being discussed may make this particular interface change moot.