Refactor (deduplicate code)

[mslw]

Currently there are four commands (export-redcap-form, export-redcap-project-xml, export-redcap-report, redcap-query).

• they all share basically the same structure, that is: a) establish paths & credentials, b) execute API query, c) write the file and datalad save. Of those, only b) is different, but a&c take more space
• they share most of their parameters (url, outfile, credential, ...) but also have a few context-dependent ones

I think from the interface point of view, it is good to keep the command separate, as they target different uses.

I've been thinking of:

• moving parameters to a separate file and importing them -- I already do this for save_message_opt and nosave_opt from datalad.interface.common_opts. But the downside is that when looking at the command code, there is less information available immediately (parameter docstrings are also kind of documentation).
• packaging a) and c) from the description above into utils - but there are many values to be passed around, and I didn't find a neat way to do it
• doing it the other way round and creating one big class from which other would derive - but again I am not sure how to do it nicely

I would appreciate suggestions from someone more experienced. Looking at any of the commands should be sufficient to get the idea.