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

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.

Start by locating the hard-coded HT9 structural/cladding material and tracing how the current backbone is consumed. Determine the additional solids needed for the three reactor types and how users should select them; done means those materials are available through the dictionary without losing existing HT9 behavior.

Written by the indexing model from the issue text.