Write new materials and access them from input files without modifying serac

[chapman39]

It would be useful if users could create their own material types and get the benefits of an input deck.

From @samuelpmish #1001 (comment)

For example (although not necessarily a good implementation), instead of a std::variant, we could have a user-extendable std::map<std::string, std::function< ... > > of callbacks. After reading the name of the material model, it would invoke the callback in that std::map (where the user could write code to set up their custom material model, and then register it to the physics module with setMaterial).