API reorganization

[tpapp]

Rething API before tagging an initial release:

• [ ] recognize both the callable and transformation as the primary transformation interface
• [ ] remove logjac, transformation_and_logjac is more practical as one would rarely ever calculate the log Jacobian without the transformation
• [ ] clarify length and the role of dimensions, perhaps a separate abstract type for transformations that map from a vector of reals?
• [ ] reconcile three kinds of APIs: one primarily for MCMC, the other for integrals, the third for stretch transformations (eg Chebyshev)