Revisit names and hierarchy

[ctb]

A few things for @lgautier and @luizirber to weigh in on --

• the sourmash_lib.signature module is badly named; I keep on colliding names with sig, etc. I was thinking of renaming it sigutils or something.

• the Estimators class is badly named. It's a legacy class anyway, held over from when MinHash didn't exist. It's still a convenient wrapper around the CPython MinHash class, at least for now, since pure Python is much easier to write, change, and test than new C code. One approach might be to deprecate the direct use of MinHash and wrap it more tightly in Estimators, and then rename Estimators to MinHash. Or if that's too confusing, MinHashWrapper. Thoughts?

• regardless of what we do with the names, Estimators should be moved out of __init__.py.

Not hugely urgent but if you have super strong opinions, now is the time :)

[lgautier]

Just opinions:

• I prefer signature to sigutils (the later bringing little IMHO - to take a parallel, I like better the stdlib package os to be called that way rather than osutils). Not saying that signature should not be named anything else though...

• I agree that Estimators is not the best name for what it represents. The C-level class could be _MinHash (or better _MinHashSketch, since this is a collection of hash values) while the Python-level wrapper be simply MinHash (or MinHashSketch). That's strategy I like because it lets one implement ideas in Python and only move what is both meant to stay and performance-critical down to C as needed.

[ctb]

The 'estimators' class was completely removed and now 'Signature' has a 'minhash' member, as of #155. We'll leave 'signature' as is, as suggested by @lgautier.