further unify moltype handling to facilitate adding additional types

[bluegenes]

Adding moltypes would be easier with additional unification of moltype handling and testing utils.

In #3446 and #3457, we added capacity for the python layer to read a new moltype, which primarily enables the sig commands for database examination and manipulation. The positional argument change needed (ref #3451) was particularly nonintuitive.

[ctb]

see comments in https://github.com/sourmash-bio/sourmash/issues/3171 as well as revamped test infrastructure in https://github.com/sourmash-bio/sourmash/pull/3627