[MRG] check for JSON in first byte of LCA database file

[ctb]

Change LCA_Database.load(...) to look at the first byte of the LCA database file, to see if it's a plausible JSON file.

This avoids costly loading of very large files by the rather dumb JSON loader we're using ;). See #1483

Other, better fixes could include -

• changing over to a different JSON loader
• doing more sophisticated checks

but this fix works fine for now!

[codecov]

Codecov Report

Merging #1495 (f6fdee3) into latest (9b5c08f) will increase coverage by 5.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1495      +/-   ##
==========================================
+ Coverage   89.77%   94.87%   +5.10%     
==========================================
  Files         123       96      -27     
  Lines       19565    15956    -3609     
  Branches     1497     1498       +1     
==========================================
- Hits        17564    15139    -2425     
+ Misses       1775      590    -1185     
- Partials      226      227       +1     

Flag	Coverage Δ
python	94.87% <100.00%> (-0.03%)	⬇️
rust	?

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

Impacted Files	Coverage Δ
src/sourmash/lca/lca_db.py	91.22% <100.00%> (-1.09%)	⬇️
tests/test_lca.py	99.87% <100.00%> (+<0.01%)	⬆️
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/encodings.rs
src/core/src/wasm.rs
src/core/src/index/storage.rs
src/core/src/index/mod.rs
src/core/tests/test.rs
src/core/src/index/bigsi.rs
... and 20 more

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[ctb]

Ready for review and merge @keyabarve @bluegenes @erikyoung85

[bluegenes]

lgtm!

neat that fp.seek(0) resets the file position at 0!

[ctb]

thanks!