MRG: add zip reading to fastmultigather

[bluegenes]

Follow up to #105:

• Enables sourmash zipfiles to be used as input to fastmultigather.

to do:

• use file loading functions from MRG: add utility functions for zip reading; use in index, multisearch #105
• modify tests as needed for unified reporting logic
• add tests for loading from zipfiles

Notes:

• When we build pathlists from zipfiles, internal sketches are extracted to individual files. To prevent excessive and unhelpful warnings, we only warn about individual input files without compatible sketches if the input was not a zipfile.
• This also means tests need to be different - sig files will be named by their md5 instead of the zipfile filename

[bluegenes]

@ctb ready for review