This function sets the value of a term that
is the time spent here, stored as TiSp_frac_visitorsing
on the XY_interface.
Here is defined as time
spent in one of the patches in this model, so
travle is time spent elsewhere.
A function F_visitors
computes TiSp_frac_visitorsing
and
this function sets TiSp_frac_here.