Clean up confusing DigitsCollection name

[marc1uk]

The WCSimWCDigitsCollection created by WCSimWCPMT is constructed in the WCSimWCPMT::Digitize function, with 2 names: "WCDigitizedCollectionPMT", declared in the Digitize() call, and "WCRawPMTSignalCollection" declared in the WCSimWCPMT constructor.

Both are passed as arguments when instantiating the WCSimWCDigitsCollection, apparently setting the 'DetName' or 'DMname', and 'ColName' of the DigitsCollection respectively.
The ColName (or collection name) can be used by the G4DigiManager to retrieve the digits collection. The purpose of the DetName variable in the DigitsCollection is unknown, and does not appear to ever be used.

The digit and hit collections are already complicated enough, and giving the same collection two names only further obfuscates things. As per other DigitsCollections, just use "WCRawPMTSignalCollection" for both ColName and DetName, to make things (somewhat) clearer.

[marc1uk]

confirm this doesn't change anything.