Fastmultigather --save-matches sig name to be identifier only

[mr-eyes]

Quoting @ctb comment

          Testing at the command line -

sourmash scripts fastmultigather 47.sig.zip 47.sig.zip -s 10000 --save-matches

I get:

NC_009661.1 Shewanella baltica OS185 plasmid pS18501, complete sequence.matches.sig
NC_009661.1.gather.csv
NC_009661.1.prefetch.csv

so I think I'd suggest splitting off the identifier as with gather.csv and prefetch.csv.

In addition, I'd like to suggest compressing the matched sketches on writing; example code to do so appears to be over in sourmash, file src/core/src/ffi/signature.rs. If you don't feel like adding it please just create an issue to do so ;).

Originally posted by @ctb in #397 (comment)