update sourmash multigather to save hash abundances to .unassigned.sig

[ctb]

In #1620, we factored out the abundance-recovery behavior in sourmash gather for --output-unassigned into a new MinHash.inflate(...) method. This abundance-recovery behavior could now easily be added to multigather, which currently doesn't recover abundances properly.

See CTB: note, multigather does not save abundances in commands.py, near line 980.

To fix:

• start by writing a test for multigather based on the gather tests for inflation, and make sure that fails on multigather;
• fix multigather's implementation to pass the test by using the inflate method

[keyabarve]

@ctb Can I work on this issue?

[ctb]

sure!

[ctb]

being tackled in #1720.