Make Transformer parameters optional and fallback to Calibrator.measured data to then gen transformer params

[jamienoss]

We want Calibrators and Transformers to be trivially configurable and thus swappable. To do this, we need to make Transfomer.Config.parameters optional such that only some config fields are populated, e.g., polyfit degree, but not the actual polynomial coefficients.

Then, a calibrator would need measured_data (relates to #165) to call Transformer.fit to fit the measured data and generate the desired transformer parameters.

If both transformer optional params and calibrator optional measured data aren't present, obviously raise.

[jamienoss]

@imaitland FYI