add working log likelihood computation to hssm class

[AlexanderFengler]

• rv_to_initial_values() is now None everywhere. PyMC complains in various places if not.
• log_likelihood computations were broken for some model formulations. after some investigation, it seems that bambi automatically wanted to compute everything in batch mode and that created some dimensionality issues. I now apply a for loop over draws and chains instead.