change the verbose param of optimize_prior! from a boolean to an integer?

[pasq-cat]

at the moment in the mlj interface the verbosity parameter accepted by mlj must be translated in a boolean through
verbose = verbosity == 0 ? false : true

In accordance with MLJ, maybe we can change the verbose param in "verbosity" and change its nature to an integer so that in future different levels of informations can be displayed depending on the verbosity level set in MLJ.
Otherwise i will keep things as they are.

btw since the training loop of optimize_prior! is not exposed i cannot continue it in the MMI.update function, only start it from zero with a new number of steps.