rename `crowsetta.validation._parse_file` -> `validate_extension`

[NickleDave]

This function is not clearly named and may not be working correctly in all cases: https://github.com/NickleDave/crowsetta/blob/a6ec84fa93762a8ab9b4b063b65a51933e010892/src/crowsetta/validation.py#L67

• [ ] it should not have a preceding underscore since there's nothing "private" about it
• [ ] _parse_file could mean anything; what it actually does is validate_extension, rename to something like that
• [ ] the function is currently called by notmat2csv and textgrid2csv; it works as intended when the input is a list of filenames. It is not called by koumura2csv or yarden2csv. Note that both use only a single annotation file to represent annotations of multiple audio files. I tried to include it in yarden2csv but got a weird error because it returns a list with just the single file name. Because it wraps any str or Path input in a list to iterate over and check. I then tried to fix this by just getting annot_path[0] only if annot_path is a list of length one, but this broke many tests so I removed it and did not pursue further. Tests for notmat2csv and textgrid2csv should test with a single file not just a list -- writing those tests might help reveal the root of the issue

[NickleDave]

I am renaming this function but I'm not going to bother making it work across all formats because I will probably just factor out what it's doing anyway

The key issue here is that the function has side effects that are not explicit; it converts a list of paths to str, and the functions that currently call it depend on this behavior for other stuff they do, i.e. to iterate over the list and be able to treat each item in it as a str

But if I'm going to represent single annotation files as classes then there won't be any need for this kind of impenetrable special-cased control flow anyway