sum_hills: reads and uses INTERVALs
GRID: I changed many plumed_assert to plumed_dbg_assert

grids are quite faster in this way, and I think that all the asserts
were redoundant with those in the initialisation.
Fixed massive bug in SPRINT

SPRINT was not applying derivatives correctly.  Rats!
Merge branch 'master' of github.com:plumed/plumed2
Merge branch 'master' into field-cvs