New families: rotation mixtures

[tnagler]

I'd like to implement the mixture models proposed here at some point. There are two options:

1. Do this in vinecopulib directly. In that case, we should start thinking about our default family set. That's maybe not such a bad idea anyway.
2. Do this in an extension library. Then we would need to come up with a mechanism that allows external libraries to extend the allowed families. Can't think of an easy way to do this though.

[tvatter]

I would like to include more families in vinecopulib. Actually, our oldest unresolved issue is exactly that #3. Ideally, we should find a MSc student to do that ^^