make the tcl command "integrate" less dependent on autoupdates

if i saw it correctly the behaviour of the integrate function depended on the whether autoupdates of observebles or correlations where enabled or not.
e.g.
integrate 10000 recalc_forces
would have recalculated the forces every time step, if autoupdates are enabled, if not, only once at the beginning.
reuse_forces is a local variable, defined in line 201, therefore it is not necessarily zero. The integrate_vv function cannot change it.