Add single-app only inputs for testing purposes

[GiudGiud]

Reason

It is often the case that the main input (for example neutronics) uses syntax from another app (combined like bluecrab or another single app like pgh).
This prevents using the regular main app for testing that input and requires us to use BlueCrab. This lags finding defects with models, and lets the app teams break our models in a very delayed manner
We have to stop this because it's a huge time sink to fix things in a delayed manner

Design

Create a _base.i to stash most of the syntax of the multiphysics case and share it with the single-physics case
Create "neutronics_only" inputs that shed any unsupported syntax from the neutronics inputs
Similar for other codes

Impact

Reduce maintenance time