fixed vector parameters should be possible #842
Description
We allow parameters to be passed as fixed float, in which case they are taken out of the graph of random variables and are just fixed in the likelihood.
Somehow we do not allow parameters to be fixed as vectors however, which is likely because it necessitated some special logic and was pushed down the priority list some time in the past.
This functionality is increasingly requested by users however and needs to move up the deck now.
@mcfreund kindly volunteered to take a look with our help.