Add doxygen documentation and minor changes

Added some (doxygen) documentation and comments to BeamFormer class.

I was just wondering whether we could:

• simplify local_arrayFactor (https://git.astron.nl/RD/EveryBeam/-/blob/7-document-beamformer/BeamFormer.h#L90) to

virtual diag22c_t local_arrayFactor() const override
{
  return {1.0, 1.0};
} 
- tighten the filling of the `result` in `local_response` a bit (i.e. the response matrix, https://git.astron.nl/RD/EveryBeam/-/blob/master/BeamFormer.cc#L101)


Closes #7