help keeping track of variable ids #116

Sometimes you have a "name" for a variable when you create it, and you want to easily go between the "name" and the variable's TypedID. Examples:

• [WIP] Chordal Initialization #108 -> ChordalInitialization.swift -> solveOrientationGraph: This takes a factor graph g as input and constructs a corresponding "unconstrained chordal graph" that has one variable per variable in g. (The variables in the "unconstrained chordal graph" have a different type from the variables in g). Then it solves the "unconstrained chordal graph" and finally it reads the solution and uses it to construct good initial values for variables in g.
• You read a factor graph from a file on disk, and the variables have string identifiers. You want to solve the graph and then write the solution to disk, using the string variable identifiers.

Having a convenient pattern/utility/something for doing this would be convenient.