Implement Joint-likelihood in `tupak.core.likelihood`

Implement joint likelihood for N-number of likelihoods. Thanks to @gregory.ashton for the example for 2 likelihoods!

class JointLikelihood(tupak.core.likelihood.Likelihood):
    def __init__(self, likelihoodA, likelihoodB):
        self.likelihoodA = likelihoodA 
        self.likelihoodB = likelihoodB
        self.parameters = self.likelihoodA.parameters
        self.parameters.update(self.likelihoodB.parameters)

    def log_likelihood(self):
        self.likelihoodA.parameters.update(self.parameters)
        self.likelihoodB.parameters.update(self.parameters)
        return self.likelihoodA.log_likelihood() + self.likelihoodB.log_likelihood()