Buffer estimate now works with rvd-switch=0

With rvdw-switch=0, the Verlet buffer estimation code would divide
by 0. The estimate for vdw-switch functions now use the exact 3rd
derivative of the potential at the cut-off iso estimating the 2nd.
grompp now prints a note with rvdw-switch < 0.5*rvdw.
Updated estimate formula in the manual and added a missing factor
of 2 (the code did use the factor of 2).

Change-Id: Ia259d703515f69b0ed3390e2f7be671e0a15bd5f