SimulatorTimer: make it possible to base it on opm-parser's TimeMap

Since SimulatorTimer is a fairly shallow shim if using the TimeMap, it
can also be removed relatively easily. Having said this, that would
trigger _many_ changes in _a lot_ of places and I'm not motivated at
all to fight that battle as long as the old parser needs to be
supported. I thus decided that the best way is to add a "wrapper mode"
to SimulationTimer...