Generalizations in output_plots.py

[jdebacker]

This PR does to things to make output_plots.py work better with different parameterizations:

1. It ensures that the num_years_to_plot is less than T (or raises an assertion if not).
2. Labels for ability types are generalized to work with any lambdas values.

Addresses Issue #757

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 79.31034% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 73.47%. Comparing base (95987b7) to head (0206cd0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #923      +/-   ##
==========================================
+ Coverage   73.44%   73.47%   +0.02%     
==========================================
  Files          19       19              
  Lines        4610     4637      +27     
==========================================
+ Hits         3386     3407      +21     
- Misses       1224     1230       +6     

Flag	Coverage Δ
unittests	73.47% <79.31%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
ogcore/output_plots.py	95.33% <79.31%> (-1.18%)	⬇️

[rickecon]

This looks good to me. Thanks @jdebacker. Merging.