[MRG] add optional abundance preservation to sourmash signature intersect

[ctb]

This PR adds an option --abundances-from/-A to sourmash signature intersect that allows the user to specify a specific signature from which to preserve abundances in the output signature.

Ref #743.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #747 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
+ Coverage   88.59%   88.62%   +0.03%     
==========================================
  Files          30       30              
  Lines        4727     4740      +13     
  Branches       46       46              
==========================================
+ Hits         4188     4201      +13     
  Misses        536      536              
  Partials        3        3

Impacted Files	Coverage Δ
sourmash/sig/__main__.py	95.32% <100%> (+0.14%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ee195b4...502eee9. Read the comment docs.

[ctb]

Ready for review @luizirber

[taylorreiter]

Looks good to me! So many new possibilities!