Add LOFAR array factor correction demo

The EveryBeam interface works somewhat differently from the old LOFARBeam library. Most notably is the use of ITRS x, y, z coordinates instead of a method which you feed right ascension and declination for e.g. the array_factor method. This documentation demo summarises one way how to calculate the array factor for LOFAR in a given ra-dec direction and may perhaps be useful for future reference.