provide a summarization command for signature collections

[ctb]

from #1349 (comment) -

@bluegenes suggests:

sourmash sig describe --> yes, would be nice to support usage, but output may not be as useful? Likely too many sigs for full output, and sourmash sig describe <zipfile> | head would no longer represent the parameters of all sigfiles, since it's possible to incorporate incompatible signatures. If enabled, maybe we could summarize the sig params (e.g. num sigs per ksize/alpha/scaled ) as tsv or tabbed terminal output? Or have a quick default option that gives info for first sig, and summarize option that gives full summary? Somewhat related to #1190.

maybe a sourmash sig summary or sourmash sig summarize command?

[ctb]

cc #1599 #1352

[ctb]

with manifests & the manifest API, this becomes much more straightforward!

[ctb]

oh! @bluegenes do you think we should alias fileinfo to summarize?

[bluegenes]

fileinfo is plenty descriptive, but sure, why not!