Python improvements + refactor

[sc1f]

This PR irons out a few errors with the Python API and makes it more consistent with the Javascript API as well as the results expected by the viewer and associated plugins.

Notable changes:

• update_callback, remove_update, on_delete, on_update have been implemented
• data accessor API updated to be consistent with JS
• bugs in to_format have been fixed
• row_pivots and column_pivots use underscores, thus following the core API

[codecov-io]

Codecov Report

Merging #730 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #730      +/-   ##
==========================================
+ Coverage   92.29%   92.37%   +0.08%     
==========================================
  Files          59       61       +2     
  Lines        2141     2703     +562     
==========================================
+ Hits         1976     2497     +521     
- Misses        165      206      +41

Impacted Files	Coverage Δ
perspective/table/view.py	80.95% <0%> (-16.07%)	⬇️
perspective/tests/table/test_update.py	88.76% <0%> (-11.24%)	⬇️
perspective/table/view_config.py	96.42% <0%> (-3.58%)	⬇️
perspective/tests/table/test_view.py	98.46% <0%> (-1.54%)	⬇️
perspective/table/table.py	91% <0%> (-1.16%)	⬇️
perspective/tests/table/test_to_format.py	100% <0%> (ø)	⬆️
perspective/table/_data_formatter.py	95.23% <0%> (ø)	⬆️
perspective/tests/table/test_table.py	100% <0%> (ø)	⬆️
perspective/table/_exception.py	100% <0%> (ø)
perspective/table/_callback_cache.py	90.9% <0%> (ø)
... and 2 more

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[timkpaine]

We'll need to go back through and update the readme and docs to consolidate the instructions for building the node and c++ into one unified set

[timkpaine]

It's all one package now

[texodus]

Thanks for the PR! Looks great!