Upgrades to support multi readout charge collection

[jgalan]

I had to update TRestDetectorSignalToHitsProcess so that it is capable to add charge to more than one TRestDetectorReadoutPlane. In general it will add charge to any readout plane definition where the hit is found inside the volume defined by cathode and readout plane positions (and the charge falls inside a readout channel).

List of changes:

• TRestDetectorSignalToHitsProcess has now been fully documented. Including image showing the process input and output event. https://sultan.unizar.es/test/classTRestDetectorHitsToSignalProcess.html

• TRestDetectorReadout::GetHitsDaqChannelAtReadoutPlane method added to check the daqId for a specific readout plane so that TRestDetectorHitsToSignalProcess iterates each plane and gets the signal id associated with each plane.

• TRestDetectorHitsEvent::DrawEvent aesthetical improvements. And a fix.

• TRestDetectorHitsToSignalProcess now returns nullptr if no signals have been found. The event will then be ignored.

This update was required by segmented mesh microbulks.