Implement arithmetic operations on `FunctionData`, etc.

[GabrielKS]

It seems straightforward, uncontroversial, and useful to implement certain arithmetic operations on FunctionData:

• Multiplication by a scalar: (c * f)(x) = c * f(x)
• Addition with another FunctionData of the same type: (f + g)(x) = f(x) + g(x)
• Addition with a scalar??: (f + c)(x) = f(x) + c

This could then be bubbled up through the cost struct layers of abstraction to the ProductionVariableCostCurve level.

[GabrielKS]

Along similar lines but we might have to define our own interface:

• Shifting by a scalar: (f >> c)(x) = f(x - c)