improve manysketch for (many) metagenome samples

[bluegenes]

current manysketch:

• only allows direct dna or protein sketching, not translation.
• input csv requires columns: name,genome_filename,protein_filename, which is fantastic for building databases and less fantastic for sketching metagenomes if you want to use both R1/R2 reads without concatenating files.

Note, this mirrors sourmash sketch fromfile template/limitations.

Here, we could try allowing an alternate fromfile input format with columns name,read1,read2 by checking this option if name,genome_filename,protein_filename header is not found or if the user provides some flag. Could we allow translate as a parameter option, too?

[bluegenes]

name,read1,read2 file input was enabled in #184!

...but we don't allow translation yet. will punt to another issue