[MRG] Improve documentation around abundance projection

[ctb]

Updates doc/classifying-signatures.md with abundance projection details.

Fixes #1041

• 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 #1073 into master will increase coverage by 8.96%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1073      +/-   ##
==========================================
+ Coverage   83.77%   92.73%   +8.96%     
==========================================
  Files          99       74      -25     
  Lines        8987     5631    -3356     
==========================================
- Hits         7529     5222    -2307     
+ Misses       1458      409    -1049     

Flag	Coverage Δ
#rusttests	?

Impacted Files	Coverage Δ
sourmash/signature.py	92.38% <0.00%> (-0.28%)	⬇️
sourmash/_minhash.py	94.69% <0.00%> (-0.27%)	⬇️
sourmash/command_compute.py	97.44% <0.00%> (-0.20%)	⬇️
sourmash/sbtmh.py	89.71% <0.00%> (-0.19%)	⬇️
sourmash/nodegraph.py	93.57% <0.00%> (-0.18%)	⬇️
sourmash/sbt_storage.py	92.00% <0.00%> (-0.12%)	⬇️
sourmash/index.py	97.61% <0.00%> (-0.09%)	⬇️
sourmash/sbt.py	88.79% <0.00%> (-0.07%)	⬇️
sourmash/lca/lca_db.py	96.20% <0.00%> (-0.06%)	⬇️
sourmash/cli/__init__.py	98.90% <0.00%> (-0.02%)	⬇️
... and 25 more

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[ctb]

Ready for review and merge @taylorreiter @bluegenes @erikyoung85, notwithstanding the test/build errors, which seem to be unrelated to the code changes.

[bluegenes]

typo, otherwise LGTM.

It might be nice to provide the full code for generating the example data you use in the example (to enable folks to run it themselves), but it's also nice to keep things focused.

[ctb]

re providing code, it's in the test comments!