Fix the design issues in MeasurableTransform

Currently, MeasurableTransform keeps a fixed set of valid_scalar_types consisting of supported scalar Ops. This needs to be completely changed so that supported Ops can be extended in a reasonable way, and without the need to import/specify them in AePPL itself, or perform hackish updates to static lists in AePPL classes.

The entire approach needs to be reconsidered carefully, although the aforementioned improvements are still a good start.