Fixed automatic backbone detection for secondary structure variables

The nature of the backbone is now specified by a class called MolDataClass.
A warning is given about GLY residues
Added functionality to read torsions for proteins using psi-4 or phi-6
Merge branch 'master' into fix-molinfo
Added colvar that measures dihedral correlation
Added regtests for dihedral correlation
Added details of how you can use phi and psi in manual for torsion and AlphaBeta
Added @ sign infront of phi and psi labels

@ sign is now used within plumed to specify special groups of
atoms or special colvars whose labels are created by plumed and
not by a user