adding THaverageSlugTemp parameter

[albeanth]

A T/H parameter that is useful for storing temperatures of the slugs/pins for non-fuel geometries.

@dlangewisch

Description

---

Checklist

• The code is understandable and maintainable to people beyond the author.
• Tests have been added/updated to verify that the new or changed code works.
• There is no commented out code in this PR.
• The commit message follows good practices.
• All docstrings are still up-to-date with these changes.

[albeanth]

Unit testing of armi/physics/thermalHydraulics is (well, seems) to be taken care of in unit testing outside of this directory. Likely in the form of reading a settings file (...somewhere...). So though there isn't a test directory, the coverage for this file is 100% before and after the changes proposed in this PR.

Within armi/physics:

> coverage run -m pytest
> coverage report -m --include=./thermalHydraulics/**
Name                              Stmts   Miss  Cover   Missing
---------------------------------------------------------------
thermalHydraulics\__init__.py         1      0   100%
thermalHydraulics\const.py            1      0   100%
thermalHydraulics\parameters.py      80      0   100%
thermalHydraulics\plugin.py          21      1    95%   50
thermalHydraulics\settings.py         9      1    89%   46
---------------------------------------------------------------
TOTAL                               112      2    98%

[albeanth]

(just for posterity)

This PR was rolled back as the parameter was quickly deemed unnecessary.

I was that guy today.