[MRG] improve error handling etc. in sourmash lca index.

[ctb]

Many LCA fixes/cleanups in reaction to challenges and opportunities from working with the GTDB taxonomy.

LCA updates:

• Add --require-taxonomy and checks for duplicate identifiers to sourmash lca index.
• Add checks for species count to taxonomy spreadsheet loading, to fix an observed heisenbug with using the wrong columns.
• add --force argument passing appropriately in response to newly failing tests.
• add several useful cached properties to LCA DBs.

Non LCA fixes:

• fix make build to always execute with a .PHONY magic dependency
• ignore matplotlib warnings during tests
• clean up ijson bytes warnings by changing use of io.StringIO to io.BytesIO

• 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 #798 into master will increase coverage by 0.11%.
The diff coverage is 95%.

@@            Coverage Diff             @@
##           master     #798      +/-   ##
==========================================
+ Coverage   79.62%   79.74%   +0.11%     
==========================================
  Files          45       45              
  Lines        6592     6630      +38     
  Branches      454      454              
==========================================
+ Hits         5249     5287      +38     
  Misses       1042     1042              
  Partials      301      301

Impacted Files	Coverage Δ
sourmash/lca/command_compare_csv.py	93.42% <ø> (ø)	⬆️
sourmash/lca/lca_utils.py	96.73% <100%> (+0.21%)	⬆️
sourmash/lca/command_index.py	91.42% <86.95%> (+0.72%)	⬆️

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[ctb]

@luizirber some more tests are needed, but I'd appreciate your quick checkover of the cached_property stuff in lca/lca_utils.py if you have a minute - am I doing it right?

[luizirber]

Other than the @cached_property comments, LGTM

[luizirber]

But tests are failing in 2.7 and 3.5, due to output order?
https://travis-ci.com/dib-lab/sourmash/jobs/269481210#L1009
https://travis-ci.com/dib-lab/sourmash/jobs/269481211#L1001

[ctb]

thanks for all the suggestions - will merge when tests pass!