[MRG] change default behavior in lca summarize

[ctb]

Two changes to lca summarize for better default behavior for 4.0 --

• Make --singleton behavior in lca summarize the default, and remove old merge-all-signatures behavior.
• Make --with-abundance the default, replace with --ignore-abundance

Fixes #929.
Fixes #1024.

Nice code simplification too :)

• 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 #1175 into latest will increase coverage by 9.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1175      +/-   ##
==========================================
+ Coverage   84.11%   93.21%   +9.10%     
==========================================
  Files          99       75      -24     
  Lines        9258     5910    -3348     
==========================================
- Hits         7787     5509    -2278     
+ Misses       1471      401    -1070     

Flag	Coverage Δ
#rusttests	?

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Impacted Files	Coverage Δ
sourmash/cli/lca/summarize.py	100.00% <100.00%> (ø)
sourmash/lca/command_summarize.py	91.26% <100.00%> (+1.33%)	⬆️
src/core/src/ffi/signature.rs
src/core/src/signature.rs
src/core/src/ffi/minhash.rs
src/core/src/index/sbt/mhbt.rs
src/core/tests/test.rs
src/core/src/ffi/nodegraph.rs
src/core/src/wasm.rs
... and 17 more

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[ctb]

ready for review!

[bluegenes]

lgtm!

[ctb]

thanks!