added DeltaR as a variable
added missing import
added protection against vectors which don't have an Eta(..) property (e.g. transverse vectors)
initial commit: a class for transverse vectors
added support for single transverse vectors
added TransverseVector.py
added support for invalid sums with TransverseVectors
initial commit: a transverse mass function
added TransverseMass function to default list at hadron colliders