Add new structural materials for cladding for molten salt reactors, sodium fast reactors, and high temperature gas reactors

[lcarasik]

We currently only have HT9 steel hard coded into TACOCAT for structural/cladding materials. We need to add more solids into TACOCAT and turn the current backbone into a dictionary to enable a user to switch the materials.

[vulcanoffj]

I would start working on implementing cladding materials such as Zircaloy-2, Zircaloy-4, Kanthal APMT, graphite, and stainless steel.

[vulcanoffj]

Add the cladding materials information based off the structure of how we tackled the fuel_props.py file.

[lcarasik]

This issue will be addressed after issue #62 has been completed to create the interface for different structural materials.