Mapper-related distmap arguments should be conditional

[magicDGS]

There are two mapper-related distmap arguments that should be conditional on the mapper used (error if provided without the mapper, and required/optional depending on the argument):

• --gsnap-output-split - I know very little about this
• bwa-sampe-args - should be used for bwa-aln, and it might be required to be sure that it is doing what the user expects