colvar → reference Relation

File in colvar	Includes file in colvar/reference
DRMSD.cpp	DRMSD.h
DRMSD.cpp	MetricRegister.h
MultiRMSD.cpp	MetricRegister.h
MultiRMSD.cpp	MultiDomainRMSD.h
RMSD.cpp	MetricRegister.h
RMSD.cpp	RMSDBase.h