Create a global Operating System variable at import time

[john-science]

I believe the only functional solution would be something like:

1. In context.py, at "import armi" time, ARMI should determine if this system is Window. Linux, or Mac, and sets a global variable: armi.context.PLATFORM.
2. If it's not Windows or Linux, we fail immediately? This seems good, but is debatable.
3. Use the variable armi.context.PLATFORM throughout the codebase.

The goal here is to remove a ton of existing (and future) duplicate catches all around ARMI:

if "darwin" in sys.platform:
    # this
elif "darwin" in sys.platform:
    # that
elif "linux" in sys.platform:
    # stuff
else:
    raise OSError("xyz")

Instead we can do:

if PLATFORM == WINDOWS:
    # this
else:
    # that

[jakehader]

An Enum / Flag might be nice for this versus a string!

[drewj-usnctech]

We have a lot of mac users here despite it not being formally supported, it works well

[john-science]

We have a lot of mac users here despite it not being formally supported, it works well

Well, that's a wrench. Hmm

[john-science]

@drewejohnson I just tried to run the ARMI tests on MacOS and 18 of the 2064 tests failed:

https://github.com/terrapower/armi/actions/runs/8898351718/job/24435316299#step:5:2441

Most of the errors were of the form:

No such file or directory: 'refSmallReactor.yaml'

If that's an easy one to solve, maybe I could add MacOS to the official ARMI CI.

[john-science]

We have a lot of mac users here despite it not being formally supported, it works well

@drewejohnson Just FYI, I just opened a PR to support Macs: #1713