Add specification version to the metadata in SigMFCollection

[GreenK173]

Solves #82.

[Teque5]

When we create a new SigMFFile we simply override the given self.VERISON_KEY with the current __specification__ since that object is derived from that spec. Similarly here we don't need to check if the version is set and just do self.set_collection_field(self.VERSION_KEY, __specification__).

This is close to what you already have, so just make a quick fix.

[GreenK173]

Fixed per above comment.

[Teque5]

Ready to merge after rebase to main.

[Teque5]

Opened #91 to address lack of testing in SigMFCollection, which I realized while reviewing this PR.