GROUP: extended documentation

I have extended the documentation to include the possibility
of using ranges of atoms with positive or negative stride.
Action With Arguments: added reference to regex in the ARG documentation
Torsion and MOLINFO: extended documentation

to include the description of the current shortcut
for dihedral angles (@phi, @psi, @omega, @chi1)
Merge branch 'belfast-tutorials'
Added more descriptions of changes to the code
Norm of average vector in volume implemented

I have added a vessel so that you can calculate the average vector
directly from a VectorMultiColvar.  This replaces the old AVERAGE_VECTOR
feature and is better because you can use this vessel in combination with
AROUND to calculate the norm of the average vector within a region of the box.
User Guide Intro Fix

The example given for continuation line was wrong.
ANGLES: fixed manual

fixed the examples
Documentation fixes
HISTOGRAM: added GRID_SPACING
HISTOGRAM: more error checking
COMMITTOR: better documentation and error checking
HISTOGRAM: more options
- accumulate standard histogram without using kernels
- accumulate frequency instead of normalised probabilties
- convert the histogram to a free energy
HISTOGRAM: small changes to have nice free energy
Fixed a few bugs in regtests
Merge branch 'master' of github.com:plumed/plumed2
Merge branch 'v2.0'
Merge branch 'master' into belfast-tutorials