Improvements to setBlockMesh

[albeanth]

#1665 brought up an interesting discussion between @mgjarrett and I.

setBlockMesh is intended to be used to force/snap the axial mesh of an assembly to some prescribed axial mesh.

The behavior of setBlockMesh is controlled through the parameter conserveMassFlag which either

1. "auto" -- conserves all of the components below the fuel block(s) as well only the fuel components within fuel blocks.
   • the intent was to support irradiation-based axial expansion of fuel
   • but is also odd because why care about what's below the fuel since irradiation-based axial expansion is only relevant for fuel
   • it feels like this was written for a former modeling paradigm that we've progressed beyond (which isn't a bad thing and won't be the last!)
2. "True" -- conserves all components
   • is not used within ARMI or any major downstream applications that I can see
3. "False" -- conserves nothing
   • is used in ARMI in a couple spots via default. it probably shouldn't be...

The two use cases that @mgjarrett and I discussed providing coverage for were for axial thermal expansion and irradiation-based expansion.

Axial Thermal Expansion

All solid components should have their masses conserved.

• for each assembly, all solid components within each block would be set to the new block height
• their number densities would get scaled by old_block_height/new_block_height

Irradiation-Based Axial Expansion

Focus be given to the conservation of mass for the fuel components only.

• Only fuel components will have their number densities changed. Results in a change in mass for all non-fuel solid components (e.g., clad and control mass increase if fuel grows)

Proposal

Given the above, I would propose that we refactor setBlockMesh for the above two use cases and control the behavior of which through a new parameter. E.g., something like

def setBlockMesh(self, newMesh: List[float], scenarioDriver: str):
    ```
    ...
    Parameters
    -------------
    newMesh
        The new mesh to snap the assembly to. 
    scenarioDriver
        Determine if newMesh is driven by thermal expansion or irradiation based expansion
    ```

@mgjarrett @keckler @onufer @jakehader thoughts?