mmirs_pipeline_taskfile: Handling of mos files

[astrochun]

Currently the code does not copy/symlink *mos.txt files for most cases. It only has a built-in feature for the fields VM. As a result, this breaks for MOS data reduction on any other computer.

[astrochun]

After discussion with @cnaw, it became apparent that the data retrieval script/tool for end users (i.e., PIs) does not provide the .msk files. Thus, what is needed for end users is an option to indicate where the file is located to create copies or symbolic links. This file is in fact generated using the mask design software, so the end user should in principle have a copy of it.