More flexibility for session processor yamls?

[baxpr]

Would be useful if

• Yaml can be specified with a github URL (specific tag or commit required)
• An optional name alternative can be given for the processor in the redcap form (v2 processors had this in the procname field)

Combined, this would let me have one github repo with say a bunch of fmriprep processors for different projects, each with a different filename, but let me use a shorter tag on xnat like just fmriprep instead of the dance of fmriprep-HOTC, fmriprep-NVS-2mm, ... that I'm doing now.

So, good idea / bad idea? Possible?

[baxpr]

Related to #471, as the longer procnames are causing path length related crashes in fmriprep.