MRG: add public n_hashes fn to signature

[bluegenes]

We need to be able to get the number of hashes in a minhash for accurate manifest writing. This function allows us to get it directly from the signature.
ref sourmash-bio/sourmash_plugin_branchwater#574

[bluegenes]

@ctb ready for review

[ctb]

Wouldn't we want these on Sketch (where size already exists)? What code are we trying to use these in? :)

I worry about the unimplemented bit, which connects to the issues discussed here: #3333

[bluegenes]

ah - things I did not realize: since SigsTrait is public, all it's functions are public, and I can actually access size. We still have the frustration of finding the first minhash(), but it is manageable without this PR. thx for review!