[DynareDev] [Trac] #100: Add exception for degenerate beta prior.

#100: Add exception for degenerate beta prior. --------------------------+------------------------------------------------- Reporter: stepan | Owner: sebastien Type: bug | Status: new Priority: major | Milestone: 4.2 Component: Core M-files | Version: Keywords: | --------------------------+------------------------------------------------- If the declared prior expectation and standard deviation are equal to 0.5, the prior density is not defined because in this special case the parameters \alpha and \beta are zero. We should add an exception for this case.