Demographics.py and build_and_test.yml

[rickecon]

This PR:

• Updates demographics.py. The first test test_get_pop_objs() in test_demographics.py was failing because the get_fert(), get_mort(), and get_imm_rates() functions were defaulting to producing the plots. And the plot functions are not working correctly right now (See this PR thread comment).
• Inserts the Mambaforge build approach into the build section of build_and_test.yml. I think this should help the CI move past that spot.

All the pytest tests pass for me locally after this PR.

(ogusa-dev) richardevans@Richards-MacBook-Pro-2 OG-USA % pytest
/opt/anaconda3/envs/ogusa-dev/lib/python3.11/site-packages/pep8.py:110: FutureWarning: Possible nested set at position 1
  EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[[({] | []}),;:]')
============================= test session starts ==============================
platform darwin -- Python 3.11.6, pytest-7.4.2, pluggy-1.3.0
rootdir: /Users/richardevans/Docs/Economics/OSE/OG-USA
configfile: pytest.ini
testpaths: ogusa/tests
plugins: xdist-3.3.1, pep8-1.0.6
collected 45 items                                                             

ogusa/tests/test_calibrate.py ....                                       [  8%]
ogusa/tests/test_demographics.py ........                                [ 26%]
ogusa/tests/test_get_micro_data.py ..............                        [ 57%]
ogusa/tests/test_income.py ............                                  [ 84%]
ogusa/tests/test_psid_data_setup.py ...                                  [ 91%]
ogusa/tests/test_run_example.py .                                        [ 93%]
ogusa/tests/test_utils.py .                                              [ 95%]
ogusa/tests/test_wealth.py ..                                            [100%]
================= 45 passed, 95 warnings in 2599.10s (0:43:19) =================

cc: @jdebacker

[rickecon]

@jdebacker. I can't get the documentation to build locally using the new ogusa-dev conda environment: jb build ./docs/book. These are the errors I get. Maybe things are not compiling anymore because I removed the default plotting functionality from those functions in demographics.py.