Update structure of tax depreciation rules

[jdebacker]

This PR updates the structure of the tax depreciation rules JSON to make it easier to account for multiple years' parameters.

On the pro side, this simplifies the JSON file.

On the con, I believe the new structure means that one cannot update the parameters for who classes of assets at once (e..g, change the depreciation method for all 5-year lived assets), rather one needs to change rules specifically for each asset type.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (9cf614a) to head (cc518ec).
Report is 25 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
- Coverage   99.91%   99.82%   -0.09%     
==========================================
  Files          11       11              
  Lines        1133     1165      +32     
==========================================
+ Hits         1132     1163      +31     
- Misses          1        2       +1     

Flag	Coverage Δ
unittests	99.82% <100.00%> (-0.09%)	⬇️

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Files with missing lines	Coverage Δ
ccc/__init__.py	100.00% <100.00%> (ø)
ccc/calcfunctions.py	100.00% <100.00%> (ø)
ccc/get_taxcalc_rates.py	98.24% <100.00%> (-1.76%)	⬇️
ccc/parameters.py	100.00% <100.00%> (ø)
ccc/utils.py	98.97% <100.00%> (ø)