Allow for mapping metadata between common metadata formats

[Teque5]

Moving issue from prior repo by @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.

Note we marked this for v2.x development. The desire is a mapping between VITA49 or X-Midas Blue for SigMF. These have been widely implemented but not integrated into this repository. It's a complex task but perhaps partial support is fairly easy.