2.5 different code locations that determine sim_length need to be collapsed into one.

[russelljjarvis]

Two and a half locations are

• model.set_stop_time()

• test.protocol (or test.params I can't remember which).

• current['DELAY']; current['DURATION'] model.inject_square_current(current)

• model._backend may also have its own set_stop_time, but its possible that this is not in conflict with mode.set_stop_time(), and works as expected.