Added find contour action to replace WillardChandler surface action

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

regtest/crystallization/rt-wcsurface/plumed.dat

src/gridtools/ActionWithInputGrid.cpp

src/gridtools/FindContour.cpp

src/gridtools/GridVessel.cpp

src/gridtools/GridVessel.h

src/multicolvar/WillardChandlerSurface.cpp

src/tools/Minimise1DBrent.h