MRIsConvert combine_surfs function

[tsalo]

mris_convert has a flag for combining surface files (combine_surfs), but the current interface doesn't allow for it. Since the current interface is just for converting between file types, and adding combine_surfs to MRIsConvert will require overhauling it a bit, I wanted to know whether it would be preferable to expand MRIsConvert or to create a new interface for the combine_surfs behavior specifically (something like MRIsCombine). Does anyone have a preference?

[effigies]

My personal feeling is that if you're willing to do the work, however you think best is fine. Just need not to break existing use.

[tsalo]

Thanks. I think it makes more sense to create a separate interface specifically for combining surfaces. Added in #1948.