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...
some cleanups for EclipseWriter

- the output=true|false parameter gets respected now
- if output is "true", it is checked that the value of the
  "output_dir" parameter is a directory (although I did not find an
  easy way to check whether it is writable short of writing a file to
  it.)
- the "output_interval" parameter gets respected as well
- the index of a written timestep is now independent of the time step
  index of the simulation: the initial solution is always 0, the first
  result written to disk is always 1 and so on... (i.e., it does not
  matter anymore how many steps the simulator needed between two writes)

these changes have been proposed by @atgeirr. Thanks!
Merge pull request #515 from andlaus/EclipseWriter_cleanups

Eclipse writer cleanups