Handling checking for OSs better

[john-science]

What is the change?

I went through all instance of sys.platform in ARMI to check that we were handling it correcty.

Why is the change being made?

The problem I found was people were doing this to test if the current OS is Windows:

"win" in sys.platform

This is because sys.platform returns "win32" for Windows, but it also returns "darwin" for MacOS.

Here is a helpful table:

┍━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━┑
│ System              │ Value               │
┝━━━━━━━━━━━━━━━━━━━━━┿━━━━━━━━━━━━━━━━━━━━━┥
│ Linux               │ linux or linux2     │
│ Windows             │ win32               │
│ Windows/Cygwin      │ cygwin              │
│ Windows/MSYS2       │ msys                │
│ MacOS               │ darwin              │
│ OS/2                │ os2                 │
│ OS/2 EMX            │ os2emx              │
│ RiscOS              │ riscos              │
│ AtheOS              │ atheos              │
│ FreeBSD 7           │ freebsd7            │
│ FreeBSD 8           │ freebsd8            │
│ FreeBSD N           │ freebsdN            │
│ OpenBSD 6           │ openbsd6            │
│ AIX                 │ aix                 │
┕━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━┙

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.