Added more regtests for MultiColvarDensity and fixed various bugs in this code

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

regtest/analysis/rt-cube/fesA1.cube.reference

regtest/analysis/rt-cube/fesA2.cube.reference

regtest/analysis/rt-cube/histoA1.cube.reference

regtest/analysis/rt-cube/histoA2.cube.reference

regtest/analysis/rt-cube/plumed.dat

regtest/analysis/rt-histo/analysis.0.fesA1.reference

regtest/analysis/rt-histo/fesA1.reference

regtest/analysis/rt-histo/fesA1u.reference

regtest/analysis/rt-histo/fesA2.reference

regtest/analysis/rt-histo/fesA2u.reference

regtest/analysis/rt-histo/fesA3.reference

regtest/analysis/rt-histo/plumed.dat

regtest/analysis/rt0/histoB.reference

regtest/crystallization/rt-wcsurface/dens.cube.reference

regtest/crystallization/rt-wcsurface/plumed.dat

regtest/crystallization/rt-wcsurface/surface.xyz.reference

regtest/multicolvar/rt-dens-average/Makefile

...t/multicolvar/rt-dens-average/analysis.0.dens1b.reference

regtest/multicolvar/rt-dens-average/config