Simplify Records class logic

[martinholmer]

This pull request separates the logic in the old Records class into two parts: the generic code involved in handling data observations that are aged each year and code that is specific to the Tax-Calculator model. The generic code is now in a new abstract base class called Data with code in the taxcalc/data.py file and tests in the taxcalc/tests/test_data.py file. The Tax-Calculator-specific code is in the revised Records class, which is now derived from the Data class (in the same way as the Policy class is derived from the Parameters class).

There should be no effect on models that use the high-level Records capabilities of Tax-Calculator because those high-level capabilities have not changed.

[codecov]

Codecov Report

Merging #2337 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2337   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          12      13    +1     
  Lines        2713    2741   +28     
======================================
+ Hits         2713    2741   +28

Impacted Files	Coverage Δ
taxcalc/parameters.py	100% <ø> (ø)	⬆️
taxcalc/taxcalcio.py	100% <100%> (ø)	⬆️
taxcalc/growfactors.py	100% <100%> (ø)	⬆️
taxcalc/data.py	100% <100%> (ø)
taxcalc/__init__.py	100% <100%> (ø)	⬆️
taxcalc/records.py	100% <100%> (ø)	⬆️

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