Automate common data processing steps

[khaeru]

This PR contains an example of how to address #24. The proof-of-concept in item.historical.scripts.T001, a pure-Python module that performs the same steps as T001.ipynb, while being more concise, faster, and making use of re-usable code in item.historical.process().

It also:

• Moves earlier (pre-2019) code for the historical database into a new submodule item.historical.legacy.

[codecov[bot]]

Codecov Report

Merging #25 into master will increase coverage by 1.41%. The diff coverage is 49.70%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   49.15%   50.57%   +1.41%     
==========================================
  Files          34       37       +3     
  Lines        1483     1568      +85     
==========================================
+ Hits          729      793      +64     
- Misses        754      775      +21     

Impacted Files	Coverage Δ
item/historical/legacy.py	0.00% <0.00%> (ø)
item/tests/test_historical.py	0.00% <0.00%> (ø)
item/historical/__init__.py	91.66% <89.65%> (+54.66%)	:arrow_up:
item/historical/cli.py	66.66% <100.00%> (+1.14%)	:arrow_up:
item/historical/scripts/T001.py	100.00% <100.00%> (ø)
item/historical/scripts/util/managers/dataframe.py	96.10% <100.00%> (+0.05%)	:arrow_up:
item/utils.py	100.00% <100.00%> (ø)

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