Implement cloud metrics into PMP

[lee1043]

Implement @mzelinka's cloud metric.

Message from @mzelinka:

"... Note that the paper documenting this just went under review, so it is possible that reviewers will demand that we change our metric. But it is a simple RMSE, so there is nothing really complicated. (I will present the science of this at the meeting on Thursday.)

The primary code is main.py. Please ignore all my awful coding and inability to optimize things. You will note that the code begins by creating xmls that point to the end-user’s local netcdf files. I’m sure the PMP handles this a bit differently. And probably this could just be replaced by xarray.open_mfdataset("path/files/*.nc", combine='by_coords'), skipping over the whole xml-generation step. I haven’t yet taking the plunge into xarray, so I only know this from the xCDAT doc that Jiwoo and others have put together (thanks for that!)."