[MRG] Improve our loading and saving and editing API for signatures & minhashes

[ctb]

Changes:

• rename 'estimators' to 'minhash' internally, after [MRG] Refactor sourmash_lib to remove the Estimators class #154;
• add a copy_and_clear method to create a new MinHash (make a new MinHash method that creates an empty minhash... #95);
• add a subtract_mins method;
• fix problem with saving/loading empty sig files;
• add convenience method load_one_signature;

• 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-io]

Codecov Report

Merging #155 into master will increase coverage by 0.11%.
The diff coverage is 89.04%.

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
+ Coverage   85.31%   85.42%   +0.11%     
==========================================
  Files          13       13              
  Lines        1879     1894      +15     
  Branches       52       52              
==========================================
+ Hits         1603     1618      +15     
  Misses        265      265              
  Partials       11       11

Impacted Files	Coverage Δ
sourmash_lib/sourmash_args.py	93.24% <100%> (ø)	⬆️
sourmash_lib/sbtmh.py	94.73% <100%> (ø)	⬆️
sourmash_lib/signature_json.py	95.36% <75%> (+0.06%)	⬆️
sourmash_lib/commands.py	89.44% <83.33%> (ø)	⬆️
sourmash_lib/signature.py	89.11% <91.17%> (+0.78%)	⬆️

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[luizirber]

👍