Add ability to fully represent units within cost objects, including FunctionData

[GabrielKS]

Right now, only one FunctionData field can be a pint.Quantity. It would be useful for the entire cost function API to be compatible with Pint units, including all FunctionData fields.

To go one step further, we might even validate the units of the FunctionData fields in relation to each other: for instance, for QuadraticFunctionData, there must exist some units Y and X such that constant_term has units Y, proportional_term has units Y/X, and quadratic_term has units Y/X^2. This is less important though.