Nuclear Partition Function Test Case

[ChrisDeGrendele]

Don and I will go through this together soon.

The main result:

Edot Ratio (PF/NOPF): 2.293683464                                                     
The estimated for 1e10k is 2.294                                                      

[ChrisDeGrendele]

This pull request contains two main things.

1. A single rate network.
2. test_pf This test is basically test_rhs run twice. Once with use_partition_factors = .true. and once with use_partition_factors = .false. We then construct the rhs and compare the two Edots. They should differ by the partition function factor of mg23 at the specific temperature. For now, I have hardcoded the solution into the code to compare to. The solution was generated using test_pf/solution.ipynb

[zingale]

closing this -- we have a test_part_func unit test now.