SimulatorTimer: rename currentTime() to simulationTimeElapsed()

because the name "currentTime()" can be mistaken for the point in
real-life time at which the simulation is run (e.g. March 11, 2014,
15:07:45.123), the _point_ in time which the simulator timer currently
represents (e.g. Jun 5, 1985, 02:33:12.345) instead of the simulator
time in seconds which elapsed since the START date
(e.g. 52633.345 s).

this rename may lead to some fallout in other modules. I'll
fix them after this PR has been merged...