Fix path handling for non-conforming datasets

[GreenK173]

This PR replaces PR #79 because I accidentally created that one on the main branch of my fork.

Fixes #78. The matter was also recently dealt with in the SigMF specification repo in issue #321 and PR #322. In particular this PR addresses the issue raised by @lgoix in this comment.

The get_dataset_filename_from_metadata function returns the full file path for non-conforming datasets, not just the file name. Furthermore an unit test verifying this, plus a couple of typos described in the above issue fixed.

[Teque5]

While rewriting your test I observed that to use a non-compliant data file we have to do something like this:

meta = SigMFFile(metadata=ncd_metadata, data_file=data_path)
# tell SigMF that the data is noncompliant
meta.set_global_field(SigMFFile.DATASET_KEY, data_path.name)

but really I think the SigMFFile should detect that data_path.suffix != '.sigmf-data' and set that key & value automatically.

We can either merge this and create another PR, or if you want you can add this improvement to this PR.

[Teque5]

Since we never want temp files to exist after testing I used tempfile and also make some simplifications.