Create a `TokenizedCells` class

[nleroy917]

There seems to be some desire for a more ergonomic class that can wrap a set of tokenized cells when tokenizing AnnData or fragments.tsv files. It would be a simple class that lets you pull out input ids, filter things down based on number of tokens in cells, etc.

[nleroy917]

Thinking about this a bit more... it can get complex since this is mostly needed in tokenize_fragments_file in gtars. Its less useful in tokenize_anndata I think. However it presents a bit a problem since:

tokenize_fragments_file is a rust function in gtars, while tokenize_anndata is a python function in geniml. Therefore they can't really share an API... I suppose we could make it in the python bindings and then just import that internally in geniml. But, its kind of overkill since we'd be maintaining a really simple object/class within rust, not python -- which isn't very ideal.

Thinking out loud here...