Debt to GDP plot labels

[jdebacker]

This PR updates constants.py to allow for more variables to be plotted as a fraction of GDP.

Assertions and error messages are added to let the user know if they enter a variable that is not in the list of variables to plot.

[jdebacker]

Related to Issue #960

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.31%. Comparing base (e799452) to head (79dcd46).
Report is 2 commits behind head on master.

Files	Patch %	Lines
ogcore/utils.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #962   +/-   ##
=======================================
  Coverage   71.31%   71.31%           
=======================================
  Files          20       20           
  Lines        4953     4958    +5     
=======================================
+ Hits         3532     3536    +4     
- Misses       1421     1422    +1     

Flag	Coverage Δ
unittests	71.31% <94.11%> (+<0.01%)	⬆️

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

Files	Coverage Δ
ogcore/constants.py	100.00% <100.00%> (ø)
ogcore/output_plots.py	95.38% <100.00%> (+0.03%)	⬆️
ogcore/parameter_tables.py	86.07% <100.00%> (ø)
ogcore/pensions.py	67.03% <100.00%> (ø)
ogcore/utils.py	87.01% <50.00%> (-0.17%)	⬇️

[rickecon]

@jdebacker. Looks good. Merging.