Small fix in user-doc compilation

If "plumed" executable is not available, sourceme.sh file is sourced
changelog
Merge branch 'v2.0'
FuncSumHills and SumHills extension

added the options to set spacing instead of nbin and
the option to translate the feses with the minimum set
to zero.

This also fix #72.

More work is needed to make FuncSumHills fully compatible
with COLVAR files (i.e. adding the possibility of doing
histogram only on selected collective variables/fields instead
of using all the fields).
SumHills: clarified that --kt is in energy units
CS2Backbone: small changes for better performacens (maybe)
CS2Backbone: rt0 reset
Merge remote-tracking branch 'origin/master' into belfast-tutorials