Move `a/b.p.detailedNDens` to internal plugin

[keckler]

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The parameter detailedNDens is defined for both blocks and assemblies in the framework. Here are both of those definitions: https://github.com/terrapower/armi/blob/1e356e7ee1756c4bde05eea1a9b9a8398e72c4f1/armi/reactor/assemblyParameters.py#L134-L147 https://github.com/terrapower/armi/blob/1e356e7ee1756c4bde05eea1a9b9a8398e72c4f1/armi/reactor/blockParameters.py#L51-L65

There are a couple minor issues with these parameters being defined here:

1. They only make sense within the context of the internal TP depletion solver, and are thus confusing to anyone that doesn't have access to that internal solver
2. These parameters are only set if a specific setting is turned on (burnxDetailedDepletion), and that setting is internal to TP's ARMI app
3. This parameter is seemingly duplicate with the other number density block parameters (nFe56, nU238, etc.) and is thus confusing

In order to disambiguate the situation, it would be great if we could add context to the parameter's description that, for instance, it is only set when burnxDetailedDepletion is turned on. In order to do that, the setting would have to be made internal.

I think there would be value in bringing this hyper-specific setting internal. The setting definition says as much in a comment, so I don't anticipate this would break anything.