Improve public API

[adlerjohn]

Partially addresses #12.

Don't export dataSquare methods, and instead only export ExtendedDataSquare methods that wrap those. Functionally mostly the same, except Row and Col now return copies, so the caller can't accidentally write to internal slices in a bad way.

[codecov]

Codecov Report

Merging #57 (3533af6) into master (5a692bf) will decrease coverage by 0.24%.
The diff coverage is 72.34%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   76.70%   76.45%   -0.25%     
==========================================
  Files           9        9              
  Lines         382      378       -4     
==========================================
- Hits          293      289       -4     
  Misses         57       57              
  Partials       32       32              

Impacted Files	Coverage Δ
bit_matrix.go	82.85% <ø> (ø)
tree.go	100.00% <ø> (+16.66%)	⬆️
extendeddatasquare.go	44.44% <60.00%> (+4.44%)	⬆️
extendeddatacrossword.go	71.42% <70.83%> (ø)
datasquare.go	93.18% <84.61%> (+0.25%)	⬆️
codecs.go	33.33% <0.00%> (+22.22%)	⬆️

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[liamsi]

Great! I think we can merge this as is. Though we should open an issue to make sure we test all public APIs (including methods introduced here) and about test coverage.