Allow all ksizes for protein signature calculation if input is protein

[bluegenes]

https://github.com/dib-lab/sourmash/blob/4aab62f65fb08044e9a43ec49331b65be8b5ae15/sourmash/commands.py#L161

Here, we check ksize and only allow ksize divisible by 3 when computing protein signatures, which is necessary when we need to translate nucleotide seq to protein seq, but not if the input is protein (--input-is-protein flag is set).

[ctb]

Fixed in #576.