Weibull prior distibution

CMR 2013 are using a Weibull prior distribution. I suggest to add it to the list of prior used by Dynare

@houtanb Could you please deal with the preprocessor part. Similar to https://github.com/DynareTeam/dynare/commit/8956bddebae3aac6c5811a858ea3f3d7f34a0c7e using weibull_pdf as a token should translate to code number 8 (the first 7 are taken)

@JohannesPfeifer done in eae9f688

@stepan-a I had been assigned this ticket. Now we duplicated the work.

@JohannesPfeifer I am sorry for that. I knew that you had been working on this (Michel also worked on this issue) but I needed this urgently to play with CMR for another project (also I did not remember that someone was assigned).

I still have to add unit tests for the Weibull random number generator.

@stepan-a Do you still plan on adding a unit rest for the random number generator? We do not have this for any other distribution.

@JohannesPfeifer Sorry I forgot to close this ticket, this was done in e53f3a67.

mentioned in issue #846