[MRG] Fix selection w/abund and manifest column type conversions

[ctb]

This PR fixes three issues -

• signature selection with abundances didn't work for several reasons - select_signatures(...) didn't take an abund keyword arg, and manifest selection didn't either;
• neither selection technique handled abund, of course!
• on manifest loading from CSV, the columns were not properly converted to integer/bool appropriately; this was instead done on selection!

[codecov]

Codecov Report

Merging #1645 (a862125) into latest (dcfd973) will increase coverage by 0.00%.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##           latest    #1645   +/-   ##
=======================================
  Coverage   81.91%   81.92%           
=======================================
  Files         112      112           
  Lines       11447    11462   +15     
  Branches     1432     1440    +8     
=======================================
+ Hits         9377     9390   +13     
- Misses       1813     1814    +1     
- Partials      257      258    +1     

Flag	Coverage Δ
python	89.32% <87.50%> (-0.01%)	⬇️
rust	66.43% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/index.py	95.31% <33.33%> (-0.42%)	⬇️
src/sourmash/lca/lca_db.py	91.29% <100.00%> (+0.05%)	⬆️
src/sourmash/manifest.py	90.26% <100.00%> (+0.74%)	⬆️
src/sourmash/sbt.py	82.05% <100.00%> (+0.04%)	⬆️

---

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 dcfd973...a862125. Read the comment docs.

[ctb]

Ready for review and merge @sourmash-bio/devs

[bluegenes]

lgtm!