Made it possible to calculate ensemble averages for spherical contours

regtest/adjmat/rt-dfg-wcsurf/mysurface.xyz.reference

regtest/adjmat/rt-dfg-wcsurf/plumed.dat

src/gridtools/ContourFindingBase.cpp

src/gridtools/ContourFindingBase.h

src/gridtools/DumpCube.cpp

src/gridtools/DumpGrid.cpp

src/gridtools/FindContour.cpp

src/gridtools/FindContourSurface.cpp

src/gridtools/FindSphericalContour.cpp

src/gridtools/GridPrintingBase.cpp

src/gridtools/GridToXYZ.cpp