Add `$copy()` to AbstractAnnData

[rcannood]

I wonder if $copy() should be a shorthand for this (just for people familiar with Python)?

Originally posted by @lazappi in #324 (comment)

The python copy method accepts a filename so it can be written to disk or be kept in memory. Should we implement this with the same interface so we can:

• filename → HDF5AnnData
• no filename → InMemoryAnnData

?

[lazappi]

I think that is reasonable. Assuming we have a Zarr backend at some point we might want to choose the file type based on the extension but that's something that can be added later.