Allow model to work with J=1

[jdebacker]

This PR updates OG-Core to work with a parameterization of J=1. This issue is noted in Issue #1057.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.59%. Comparing base (ab0497d) to head (0d20376).

Files with missing lines	Patch %	Lines
ogcore/txfunc.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1062      +/-   ##
==========================================
- Coverage   72.62%   72.59%   -0.03%     
==========================================
  Files          21       21              
  Lines        5096     5098       +2     
==========================================
  Hits         3701     3701              
- Misses       1395     1397       +2     

Flag	Coverage Δ
unittests	72.59% <62.50%> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
ogcore/SS.py	72.82% <ø> (ø)
ogcore/pensions.py	67.76% <100.00%> (-0.12%)	⬇️
ogcore/tax.py	100.00% <ø> (ø)
ogcore/txfunc.py	46.36% <40.00%> (-0.06%)	⬇️

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