Nexus example for QE-QMC AFM/FM solid state calculation requested

**Is your feature request related to a problem? Please describe.**

Nexus example for QMC on a simple magnetic system using DFT+U QE-based trial wavefunction and AFM+FM states for e.g. computing the exchange coupling. DFT+U should be specified using the latest QE format.

**Additional context**

This was requested in the April Office hours.

Example would clearly have to be underconverged to keep the cost minimal, but would serve as an example on how to set things up.