filter-seqs: filter by seq length

[jairideout]

Improvement Description
It'd be useful to have a way to filter sequences based on length -- maybe even an option to trim to a length, but that might be better as its own method.

References
Came up on the forum.

[nbokulich]

forum xref

[nbokulich]

The RESCRIPt plugin has a couple actions for filtering sequences by length. So perhaps this issue can be closed as not needed any more? @lizgehret ?

[lizgehret]

that makes sense to me @nbokulich!