Merge remote-tracking branch 'origin/v2.3' into v2.3

regtest/basic/rt45-mpi/atom_forces.0.reference

regtest/basic/rt45-mpi/atom_forces.1.reference

regtest/basic/rt45-mpi/atom_forces.2.reference

regtest/basic/rt45-mpi/atom_forces.3.reference

regtest/basic/rt45/atom_forces.reference

regtest/basic/rt45/colvar.reference

regtest/basic/rt45/plumed.dat

regtest/basic/rt75/COLVAR.reference

regtest/basic/rt75/Makefile

regtest/basic/rt75/config

regtest/basic/rt75/deriv-jccg.reference

regtest/basic/rt75/deriv-jhahn.reference

regtest/basic/rt75/deriv-jhan.reference

regtest/basic/rt75/deriv-jncg.reference

regtest/basic/rt75/peptide.pdb

regtest/basic/rt75/plumed.dat

regtest/basic/rt75/trajectory.xtc

src/bias/Metainference.cpp

src/colvar/CS2Backbone.cpp

src/colvar/Jcoupling.cpp