Intermediate class of variable for all flow solver variables + Fix for MUSCL in NEMO solver

[pcarruscag]

Proposed Changes

CVariable -> CFlowVariable -> CEulerVariable -> CNSVariable

Likewise for incompressible and NEMO.

CFlowVariable stores, allocates, and gives access to, primitives, their gradients and limiters, and a few other odd things.
For now this is just cleanup but it can be used to give non-virtual access to these flow variables to other solvers.
For example, the scalar solver #1330 can static_cast or dynamic_cast the nodes of the flow solver to access some quantities more efficiently.
I'll leave that for after #1330 is merged.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution is commented and consistent with SU2 style.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.

[WallyMaier]

@pcarruscag LGTM! Thanks for putting in the work and abstracting more of the repeated routines.

[TobiKattmann]

Thanks @pcarruscag 💐 Some generalization is always nice. I only a have a few minor points.

I'll try to finish #1330 (template the ScalarLayer on the specialized type of variable), so this can be merged