Add support to DeRichPD base class to allow access to the pixel areas.
Useful in the reconstruction to allow adaption to the HPD/PMT differences in a transparent way.
@seaso Please can you check this is correct. I am sure HPDs are OK but please could you check PMT (where are a little more complicated due to the different sizes) are correct.
This will allow me to adapt to the HPDs and PMTs in the RichFuture algorithms in a neat, transparent way.