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 73.20%. Comparing base (e0d0972) to head (d4935b3).

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1062      +/-   ##
==========================================
- Coverage   73.23%   73.20%   -0.03%     
==========================================
  Files          21       21              
  Lines        5178     5180       +2     
==========================================
  Hits         3792     3792              
- Misses       1386     1388       +2     

Flag	Coverage Δ
unittests	73.20% <62.50%> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
ogcore/SS.py	81.05% <ø> (ø)
ogcore/pensions.py	68.23% <100.00%> (-0.12%)	⬇️
ogcore/tax.py	99.33% <ø> (ø)
ogcore/txfunc.py	46.36% <40.00%> (-0.06%)	⬇️

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