compute backend: phoebai

[kecnry]

This PR implements an alternate backend for the phoebai backend (in progress)

TODO:

• compute checks if any parameter outside bounds of training set
• cache weights so file is only parsed once
• ensure other datasets disabled (test mult compute to fit alongside dynamical RVs)
• solver checks if fitting any parameter that does not affect parameterization
• test handling of pblum+l3 etc
• profile when running in MCMC and try to improve
• solver_times to always match native sampling to avoid double interpolation
• solvers: if using native parameterization, skip running constraints (or maybe allow only running constraints needed for the parameterization of a specific backend... need to also account for priors and checks)
• document
• test