ARMI should not forcibly add a SFP to the Reactor

[john-science]

Right now, ARMI forcibly adds a Spent Fuel Pool (SFP) to every Reactor object:

https://github.com/terrapower/armi/blob/5a41b25fe9f262acb03347923443c4a2808837db/armi/reactor/reactors.py#L213-L214

I think this is rather a lazy idea and default, meant to temporarily help out a small number of users downstream who had blueprints that didn't have SFPs in them.

I think it's time to pull the Band-Aid™ off, and make people define their own SFP in the blueprints.

And, not for nothing, I just hate the default values that are in our default SFP:

https://github.com/terrapower/armi/blob/5a41b25fe9f262acb03347923443c4a2808837db/armi/reactor/blueprints/init.py#L558

https://github.com/terrapower/armi/blob/5a41b25fe9f262acb03347923443c4a2808837db/armi/reactor/assemblyLists.py#L108-L110