Fixing parameters should be done before calling transforms

[ThibeauWouters]

In the current implementation in Jim, there is support to fix some parameters to values specified by the users. In jim.py these are added at the likelihood call here, which appears right after the transforms are called. However, in case the transforms depend on parameters which are fixed, this will throw an error. Therefore, the fixed params should be added at the start of the likelihood call.

[tsunhopang]

In addition to moving the order, one would need to i) ignore the fixed parameter dimension in the Jacobian calculation and ii) assign a random yet fixed value for its log prior.