Issues with `DistributionsUserDict`

[rmnldwg]

There's a number of issues with the current implementation of the AbstractUserDict subclass that need to be addressed:

1. The method set_distribution_params() is outdated and expects every distribution in the dict to have only one parameter
2. Also, it is not consistent with all the set_params() methods in the way it handles incoming parameters
3. The syncing function in the bilateral.py module initiates only a one-way sync between linked instances of the dict.