WIP: add sig cat utility

[bluegenes]

A version of sourmash sig cat. At the moment, I think this is not faster?? Would probably need to read multithreaded with piz to get faster. Is there any way to avoid loading the sig unless we need to down sample? (e.g just copy file?)

To avoid fail if any collection contains no signatures compatible with the desired parameters, I added a --force parameter that stops load_collection from automatically exiting if there are no signatures to load. I added a corresponding check to sigcat to ensure we have signatures to concatenate prior to starting the output zipfile (bail if not).

If --scaled selection is used, we downsample where possible to return the desired scaled.

usage:

sourmash scripts sigcat  <sigfile_1> <sigfile_2> ... <sigfile_n> -o <output>.zip

usage:  sigcat [-h] [-q] [-d] -o OUTPUT [-k KSIZE] [-s SCALED] [-m {DNA,protein,dayhoff,hp}] [-f] signatures [signatures ...]

concatenate signatures into a single sourmash zip file

positional arguments:
  signatures            sourmash signature files

options:
  -h, --help            show this help message and exit
  -q, --quiet           suppress non-error output
  -d, --debug           provide debugging output
  -o OUTPUT, --output OUTPUT
                        output zip file for final signatures
  -k KSIZE, --ksize KSIZE
                        k-mer size at which to select sketches; no default
  -s SCALED, --scaled SCALED
                        scaled factor at which to do comparisons; no default
  -m {DNA,protein,dayhoff,hp}, --moltype {DNA,protein,dayhoff,hp}
                        molecule type (DNA, protein, dayhoff, or hp; no default)
  -f, --force           force: allow input sig files to contain no signatures or only incompatible signatures