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 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.20%. Comparing base (e0d0972) to head (30450ff).

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% <66.66%> (-0.03%)	⬇️

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

Files with missing lines	Coverage Δ
ogcore/SS.py	81.05% <ø> (ø)
ogcore/__init__.py	100.00% <100.00%> (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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[jdebacker]

All tests pass locally with the exception of the test_txfunc.py test that returns a dask error (same as on PR #1084):

tests/test_SS.py ..........................................                                                [  6%]
tests/test_TPI.py .............................                                                            [ 11%]
tests/test_aggregates.py .........................................                                         [ 18%]
tests/test_basic.py ....                                                                                   [ 19%]
tests/test_demographics.py ................                                                                [ 21%]
tests/test_elliptical_u_est.py .......                                                                     [ 23%]
tests/test_execute.py .                                                                                    [ 23%]
tests/test_firm.py .....................................................................                   [ 34%]
tests/test_fiscal.py .........................                                                             [ 38%]
tests/test_household.py ...................................................                                [ 47%]
tests/test_output_plots.py ...............................................                                 [ 55%]
tests/test_output_tables.py ..............                                                                 [ 57%]
tests/test_parameter_plots.py ........................................                                     [ 64%]
tests/test_parameter_tables.py .......                                                                     [ 65%]
tests/test_parameters.py ..............                                                                    [ 67%]
tests/test_pensions.py .................................                                                   [ 73%]
tests/test_run_example.py ..                                                                               [ 73%]
tests/test_run_ogcore.py .                                                                                 [ 73%]
tests/test_tax.py ...................................                                                      [ 79%]
tests/test_txfunc.py ............................F.                                                        [ 84%]
tests/test_user_inputs.py .........                                                                        [ 86%]
tests/test_utils.py ...................................................................................    [100%]

@rickecon This PR is ready for review.

[rickecon]

@jdebacker. Thanks for this. It looks good to me. Merging now.