Added method to calculate ensemble averages

regtest/analysis/rt-average/Makefile

regtest/analysis/rt-average/ala12_trajectory.xyz

regtest/analysis/rt-average/colvar.reference

regtest/analysis/rt-average/config

regtest/analysis/rt-average/plumed.dat

src/analysis/Average.cpp

src/analysis/AverageVessel.cpp

src/analysis/AverageVessel.h

src/gridtools/ConvertToFES.cpp

src/gridtools/FourierTransform.cpp

src/gridtools/InterpolateGrid.cpp

src/multicolvar/MultiColvarDensity.cpp

src/vesselbase/ActionWithAveraging.cpp

src/vesselbase/ActionWithAveraging.h