Add an option to compress HDF5 files

[allyhawkins]

Closes #244

This PR adds new options to the sce_to_anndata() function to allow for compression of the exported HDF5 files.

• I added a compression option and set the default to gzip. I like the idea of having a separate argument rather than just using ..., partially as a reminder. I did not use the same default of none that the zellkonverter::writeH5AD function uses. Instead, I set it to gzip since I think we want to compress by default. I'm on the fence on whether that was a good decision.
• I added ... as an option to pass any additional arguments to the function.
• I updated the tests to test that compression fails with an invalid option and a test to make sure using additional arguments works. Let me know if I should create any additional tests.
• I also allowed for the .h5ad extension.

[jashapiro]

Just a couple little comments, including a reminder about match.arg, which is built for situations like this!