[WIP] Make more diagnostics accessible via the PyFBU class

[Revile77]

This PR add a few extra configurable options and diagnostics:

• Ability to set sampler initialization method (the default value is with respect to NUTS sampler
• Printout maximum a posteriori minimization steps in verbose mode
• Access to full trace object from PyMC (not just the pre-formatted trace dictionary we already provide with subset of information) in __trace__ variable
• Summary information in a pandas dataframe trace_summary from the PyMC summary method