Revise ppp.py script so that it does not crash

[martinholmer]

This pull request refactors the ppp.py script so that it does not stop with a fatal error. The refactoring does not change the logic of the ppp.py script.

Also, I added decimal points to the new float parameter recently added because ParamTools writes this parameter's values as floats (instead of integers). These changes are purely cosmetic, but will avoid spurious changes to the policy_current_law.json file when the ppp.py script is executed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (598f1d6) to head (a8ac3d0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2725      +/-   ##
==========================================
+ Coverage   98.54%   99.42%   +0.87%     
==========================================
  Files          14       13       -1     
  Lines        2616     2593      -23     
==========================================
  Hits         2578     2578              
+ Misses         38       15      -23     

Flag	Coverage Δ
unittests	99.42% <ø> (+0.87%)	⬆️

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see 1 file with indirect coverage changes

[jdebacker]

Thanks for the updates, @martinholmer , merging.