[Gridoperator-OneStepExplicit] Fix bug regarding time setting for explicit methods

Explicit time steping discretization worked properly only for problems du/dt = ... but for the problems d(f(t)u)/dt = ... not. If the term in temporal derivative is time-dependent (excepting u), time must be set to the current stage before evaluating jacobian.

E.g. Explicit Euler discretization should be (f^{n+1}u^{n+1}-f^{n}u^{n})/dt = ..., but in the previous implementation it was  (f^{n}u^{n+1}-f^{n}u^{n})/dt.
[localoperator][twophaseccfv]

In some situations and on some architecture w_l can be evaluated as zero. This avoids unexpected behaviour of the localoperator.