Simplify `ExperimentalDesign` classes by consolidating relevant properties into `idata` object

[drbenvincent]

At the moment all the Bayesian ExperimentalDesign classes have an unruly set of idata properties. In an ideal world we would be able to consolidate all these into a single idata object.

Note that this is mirrored across both the Bayesian and scikit-learn classes, so we need to keep these in line with each other.