WIP: #51 fedapi credential checks

This is a draft version of the code for #51

Improved credential checks in the GeniStdMA and GeniStdSA (the "Federation MA/SA APIs"). Instead of lots of copy-past reuse, most moethods now call GeniStd::RequireAuthorizationForSlice and GeniStd::RequireUserAuthorization GeniStd::RequireAuthorizationForSlice should now support multiple credentials, finding the needed slice credential in the list.

This is completely untested. I'll get back to this when I have tested it.

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