Allow for mapping metadata between common metadata formats

[jsallay]

SigMF is not the only description format in town and people frequently use other formats in conjunction with it, such as VITA49. Other formats will use different names for metadata, but there should in general be a 1-1 mapping between them and SigMF. It would be valuable to users to provide mappings for common formats so that a user doesn't have to reinvent the wheel whenever they want to move between formats. This would also be very useful in creating standardized blocks that convert to/from SigMF and other formats.

Additionally, users may deal with proprietary metadata formats. It would be useful to create a mapping "schema" so that they can document the translation from their format into SigMF.

[bhilburn]

In principle, I think this is a useful idea, and is generally aligned to our goal of "if there is something lots of people will want to do, try to do it canonically so there is consistency". Thinking through it, though, I think there is a point here that there isn't an existing mechanism by which a user could specify these translation tables -- the structure of an extension doesn't really work.

I think this is a useful feature but don't want to hold v1.x for it. I think it could be implemented without breaking backcompat, so labeling it for v2.x isn't totally accurate, but dropping it there for now and will re-triage once we get through our release.

[Teque5]

Moved to new python repo.