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!