Proto-Function definitions and function spaces, with a smaller example

[balacij]

Builds on #2696
Closes #2418
Relates to #2554
Contributes to #2371

• Creates FuncDefn containers -- prototype function definitions, not yet used in code generation
  • First-order functions, using curried inputs
• Creates FunctionalModel ModelKinds -- a new variant of models for defining functions
• Convert DblPendulum's angular displacement IM into a FunctionalModel as a prototype

Leaving this as a draft because I'm unsure if we want named arguments in Expr/Function definitions.

Should we work on code generation next while converting more models? I suspect that we'll need to carry a bit more information about the inputs in the FuncDefns.

[JacquesCarette]

So I really expected more changes to Expr and fewer the chunks for adding functions?

[JacquesCarette]

This is looking good now.

[balacij]

Thank you!