GPUize posterior transformations

[dominic-chang]

The ascube and asflat posterior transformations are currently not gpu friendly. These transformations could potentially be made gpu friendly and non allocating. Here's a series of helpful places to look:

• TransformationVariables.jl
  TransformationVariables is the underlying dependency used to handle the posterior transformations.
• ComponentVectors.jl
  The current representation of the data is in terms of named tuples. Representations in terms of componentvectors are likely to be more GPU friendly