new change to have multiple mask values

[dngoldberg]

this builds on PR 133 which has now been merged.

according to 133, a mask is provided which is either zero or larger than zero.

this new functionality allows a mask with multiple (positive) integer values, and a run only calculates VAF where the mask is equal to a given integer code.

the code is set via "qoi_vaf_mask_code" (integer, default 1)
and if "qoi_vaf_mask_usecode" is True, this functionality is used -- otherwise the default functionality, that VAF is calculated where the mask if positive.

the appropriate section of the toml file will be as follows:

[io]
..
vaf_mask_data_file = ...  # variable in netcdf file needs name "vaf_mask"

..

[errorprop]

qoi = 'vaf'
phase_suffix = ...
qoi_apply_vaf_mask = true       # this should be set to true to enable masking
qoi_vaf_mask_usecode = true  # if set to true, then VAF is calculated only in region of vaf_mask_data_file 
                             # where vaf_mask is equal to qoi_vaf_mask_code
                             # if set to false (default), then VAF is calculated in region of 
                             # vaf_mask_data_file where vaf_mask > 0
qoi_vaf_mask_code = 3