Add some other common likelihood function into core

It would be nice to have a some other "common" likelihood functions in the tupak core likelihoods in addition to the Gaussian likelihood, e.g.:

• Poisson likelihood (see #148 (closed) and !132 (merged))
• Exponential likelihood (see !144 (merged))
• Student's t-likelihood (see !147 (merged))
• half-Gaussian
• Gamma distribution
• Cauchy distribution
• ...