`powerMultiplier` is poorly named

[john-science]

This (wildly used) Core property powerMultiplier is poorly named:

https://github.com/terrapower/armi/blob/e2dba79a4e68c23a0dd06b90ea2e4cbaca335095/armi/reactor/reactors.py#L374-L384

I think this should be renamed symmetryFactor.

This will break the API and will require changes in several downstream repos. But that's fine, IMO.

[onufer]

Note that Blocks and assemblies have symmetry factors, and their symm factors may be inconsistent with the power multiplier

it might be confusing to have the symmetry factor be 3 on a reactor but in most assemblies and blocks its 1 (except the center assembly that its 3)

(i think this is the origin of the difference in name)

[john-science]

(i think this is the origin of the difference in name)

Well, that's fair. Do you think I shouldn't make the name change? What's your gut-check opinion?