VAF masking

[dngoldberg]

This P/R introduces a way to mask the VAF calculation done in solver.comp_Q_vaf() by introducing a logical mask.

The mask (variable vaf_mask) can be applied by setting:

• params.error_prop.qoi_apply_vaf_mask = True
• params.io.vaf_mask_data_file to a netcdf file

the netcdf file must have x and y coordinates and the mask variable should be zero where the VAF should not be calculated and greater than zero (e.g. 1) where VAF is to be calculated, and should have a variable name given by params.io.vaf_mask_field_name (default "vaf_mask")

[jrmaddison]

Seems fine, better to indent with 4 spaces. I'd prefer setting var_mask to None rather than reading params, but I see this pattern is consistent with other code.