McEventSelector: Use unsigned for RunNumber, etc properties

I discovered that the McEventSelector uses a signed int for properties like the run-number but more or less immediately casts it to an unsigned variable. There are even Property-Verifies use to ensure the value doesn't go negative. Why not using an unsigned number as property?

cc @ewelina and @averbyts in case I am overlooking something

PS: I discovered this in the context of LAr Electronic Calibration where we use the McEventSelector to process conditions (eg non-events). I wanted to use cool::ValidityKeyMax to be sure I am processing the latest input but this actually corresponds to 0xFFFFFFFF as run number.