MRG: remove BuildParams, filter via manifest / Select approaches

[bluegenes]

This PR replaces the BuildParams manual hashing + filtering with manifest filtering and selection (e.g. of moltypes) via a Select framework. This is more in tune with the approaches in sourmash core. I did add MultiSelect here, since we want to keep all templates that match sets of selection parameters.

Internal improvements to the framework introduced in 0.4.0:

• All of the parameter parsing + checks from BuildParams/BuildParamsSet is now in BuildRecord/BuildCollection. We now parse the parameter string directly into a BuildCollection, rather than going through BuildParams as an intermediary.
• To manage finding and filtering existing signatures, we now select on the BuildCollection directly via MultiSelect, e.g. for moltype filtering. We can also filter a manifest with another manifest by using PartialEq for BuildRecords. This replaces the prior approach of keeping a BuildParamsSet and hashing BuildParams manually for comparisons.
• We continue to use sourmash core ComputeParameters to actually build template signatures.
• Instead of handling DNA, protein as separate collections, manage them jointly by allowing selection on moltype and adding to DNA sigs when we have DNA, prot sigs when we have proteins. This reduces complexity of use and has the added benefit of easier addition of translated sigs if we want to add that in the future.
• Fixes consider using sourmash core ComputeParameters instead of BuildParams #113

[bluegenes]

@ctb ready for review.

This restructure happened because I was doing some manual hashing of BuildParams, and realized that manifest filtering/PartialEq has that all baked in, and manifest filtering + selection would be much more in line with sourmash core approaches. No functionality changes here, all internal updates as described above.