Resolve Issue 161

[pawel-czyz]

This PR aims to resolve #161.

Tasks:

• [x] Update the JointDistribution class.
• [x] Update multivariate normal constructor.
• [x] Update multivariate Student constructor.
• [x] Update product distribution constructor.
• [ ] Check if BMMSampler has to be adjusted.
• [ ] Update the transform (bending) function.
• [ ] Update the mixture function.
• [ ] Update the tutorial using discrete-continuous variables. Casting the Bernoulli distribution to float is not further needed.
• [ ] Further check if we manually create JointDistribution somewhere. Then, update it.

Help highly appreciated! :slightly_smiling_face: