Investigating the `get_likelihood` method

[fealho]

The get_likelihood method below may not work as expected:

https://github.com/sdv-dev/Copulas/blob/dfe3bb9cc035c8d4880724db69c86e749178e65b/copulas/multivariate/vine.py#L246-L258

An explanation of what this method does should be added, and we should investigate whether it works as expected.