move `signature::select` downsampling to `minhash()` or `get_sketch()`?

[bluegenes]

At the moment, we use signature::select to downsample minhashes if needed. However, this means we load the minhash during the select, which may happen during sig_for_dataset, for example, and then discard it, returning just the signature. That means the minhash needs to be loaded again by the user.

We can avoid loading twice by moving downsampling into the.minhash() or .get_sketch() methods instead. What do you think @luizirber?

the downside is that there are other ways to get the minhash, e.g. .sketches()[0]^, and those wouldn't have the downsampled signature even after having run select on the signature.

^ I know we're thinking of deprecating sketches, but there probably other ways too?