Revisit starttime in boundaries, forcing terms and file_function.

[ole]

Ted Rigby discovered that a hydrograph used with Transmissive_Momentum_Set_stage didn't use the real model time when domain.starttime > 0

Changing domain.time to domain.get_time in the evaluate method solves the problem, but opens the question about the general architecture. Currently, relative times are passed in (e.g. file_boundary) but changing this will touch many modules and will have to be done carefully.

The overriding principle, though, should be that whenever a modeller references time it should be the absolute time (internal time + starttime) just as we intend with spatial coordinates.