BxDecay0 validation

[gipert]

We need some BxDecay0 unit tests and validation plots. For the latter, we could start by plotting the electron summed energy distribution for the most important Ge76 decay modes implemented by BxDecay0. I would also like to see that the angular correlations are correctly implemented, so it would be useful to at least plot the distribution of angles between the two electrons in 2vbb.

There are of course many other plots that could be generated but let's start with the basics.

[EricMEsch]

There is also currently a bug, when setting up the VertexGenerator before /RMG/Generator/select BxDecay0

This is because the constructor of BxDecay requires the pointer to the VertexGenerator. I am not sure if there is an easy fix, i will look into it.

(this bug must have been around since the beginning, but no one noticed)

[EricMEsch]

Adressed in #457.
If further validation plots are of interest this issue can be re-opened or a new one can be made.