Support for fitting parametric margins

[tnagler]

Probably wrapping around https://github.com/JonasMoss/univariateML

But let's keep that for the next release.

[dmi3kno]

Is there a way to relax the requirement that only stats::Distribution can be used in vine_dist() specification of marginals? Would it be possible to look up in the environments and call user supplied function, assuming it is loaded, in the format "mypackage::qdist". Alternatively, if you need both QF and CDF you could either infer the CDF function name from the name of the quantile function or take one more argument.

[tvatter]

Hmmm, that's actually a good idea. @tnagler what do you think?