Add additional pension systems

[jdebacker]

This PR adds 3 additional pension systems (in additional the default US Social Security system):

1. A defined benefits system
2. A notional defined contribution system
3. A points system

[jdebacker]

@rickecon I'm running the full suite of tests locally tonight. If those pass, I believe this PR is ready to merge.

[jdebacker]

With the latest commits, I only have 4 test failures locally:

FAILED tests/test_txfunc.py::test_txfunc_est[DEP] - assert False
FAILED tests/test_txfunc.py::test_txfunc_est[GS] - assert False
FAILED tests/test_txfunc.py::test_tax_func_loop - assert False
FAILED tests/test_txfunc.py::test_tax_func_estimate - assert False

These are expected as we know the optimizer for the tax function estimation returns difference results with difference version of Python/SciPy.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 68.19788% with 90 lines in your changes missing coverage. Please review.

Project coverage is 71.40%. Comparing base (dc0860a) to head (f3b21f2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #946      +/-   ##
==========================================
- Coverage   71.97%   71.40%   -0.57%     
==========================================
  Files          19       20       +1     
  Lines        4732     4949     +217     
==========================================
+ Hits         3406     3534     +128     
- Misses       1326     1415      +89     

Flag	Coverage Δ
unittests	71.40% <68.19%> (-0.57%)	⬇️

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

Files	Coverage Δ
ogcore/SS.py	71.98% <100.00%> (ø)
ogcore/__init__.py	100.00% <100.00%> (ø)
ogcore/aggregates.py	100.00% <100.00%> (ø)
ogcore/constants.py	100.00% <ø> (ø)
ogcore/demographics.py	56.58% <ø> (-0.39%)	⬇️
ogcore/tax.py	100.00% <100.00%> (+0.54%)	⬆️
ogcore/pensions.py	67.03% <67.03%> (ø)

[rickecon]

@jdebacker. I reviewed this PR and everything looks good. I just submitted a PR to your branch that updates the version number of OG-Core in setup.py and in ogcore/__init__.py and adds numba to setup.py and requirements.txt. As soon as you make a decision on that PR, we can merge this.

[jdebacker]

@rickecon I just merged your PR. Thanks for thinking of those housekeeping items.