Remove visible_columns from DataPanel

[seyuboglu]

DataPanels no longer rely on visible_columns to create views. This PR removes visible_columns entirely.

Other changes:

• Improve code coverage
  • Reactivate provenance tests
  • DataPanel batch tests
  • Concat tests
  • Merge tests
• Remove Identifiers, Splits and Info from DataPanel and AbstractColumn

[codecov-commenter]

Codecov Report

Merging #125 (1c5a787) into dev (647c6d3) will increase coverage by 6.01%.
The diff coverage is 92.98%.

@@            Coverage Diff             @@
##              dev     #125      +/-   ##
==========================================
+ Coverage   77.31%   83.32%   +6.01%     
==========================================
  Files          48       46       -2     
  Lines        3015     2807     -208     
  Branches      515      477      -38     
==========================================
+ Hits         2331     2339       +8     
+ Misses        550      366     -184     
+ Partials      134      102      -32     

Flag	Coverage Δ
unittests	83.32% <92.98%> (+6.01%)	⬆️

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

Impacted Files	Coverage Δ
meerkat/mixins/mapping.py	78.18% <ø> (+6.51%)	⬆️
meerkat/mixins/cloneable.py	95.23% <66.66%> (-2.20%)	⬇️
meerkat/pipelines/entitydatapanel.py	71.62% <80.00%> (+0.78%)	⬆️
meerkat/ops/concat.py	94.59% <85.71%> (-5.41%)	⬇️
meerkat/block/manager.py	96.89% <92.85%> (-0.84%)	⬇️
meerkat/datapanel.py	91.19% <97.43%> (+16.95%)	⬆️
meerkat/block/numpy_block.py	93.42% <100.00%> (+0.66%)	⬆️
meerkat/block/pandas_block.py	96.96% <100.00%> (+0.14%)	⬆️
meerkat/columns/abstract.py	83.51% <100.00%> (+0.17%)	⬆️
meerkat/columns/numpy_column.py	87.61% <100.00%> (+0.11%)	⬆️
... and 8 more

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[seyuboglu]

Closes #65