Support for Multiple Recordings in Archives

[jhazentia]

This is a draft PR for discussion only. This is a simplified PR compared to #1. It does not include changes for collection. Tested in scos-sensor using mock signal analyzer. See below for proposed PR message to merge into the upstream repository:

This pull request is intended to add support for multiple recordings in archives. Below is a summary of the changes:

• Added README examples for loading a SigMF archive with multiple recordings, creating a SigMF archive, and creating SigMF archives with multiple recordings
• Modified SigMFArchive to accept multiple SigMFFiles
• Renamed SigMFArchive name to path
• Modified SigMFArchive to append to existing tarfile when a fileobj is passed that references an existing, open tarfile. Note that, if desired, this can be changed back to always overwrite existing file.
• Added pretty parameter to SigMFArchive to control pretty printing of SigMF metadata in archives
• Added name parameter to SigMFFile. SigMFArchive will use the SigMFFile name parameter to create recording parent directories/file names in archive
• Renamed SigMFArchiveReader name to path
• Modified SigMFArchiveReader to read archives containing multiple recordings. The __len__(), __iter__(), and __getitem__() operate on the list of SigMFFiles instead of individual SigMFFile
• Modified SigMFMetafile dump() and dumps() methods to append newline to the JSON to make this behavior consistent throughout the code.
• Added __eq__() method to SigMFFile for testing
• Misc other supporting changes and docstring updates.
• Added supporting tests

Note that this PR does not include any changes to include a collection in archive, read collection from archive, or to update gui.py.

These changes are based on my colleagues' (Doug Anderson and Todd Schumann) changes: https://github.com/ntia/sigmf/tree/multi-recording-archive.

[aromanielloNTIA]

Looks good, this is really close to ready. I have no suggestions/edits for the proposed PR message above. See a few comments to consider inline below.

[dboulware]

Consider in-line comments. There may be reasons why they shouldn't be made or it won't work, but I think it is worth considering.