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!
Made single-argument constructors explicit.

Avoids unintended implicit conversions.
Merge pull request #522 from atgeirr/explicit-constructors

Made single-argument constructors explicit.
Merge pull request #515 from andlaus/EclipseWriter_cleanups

Eclipse writer cleanups
endpoint scaling: invert condition

yeah, true and false are difficult things sometimes...
Merge pull request #526 from andlaus/endpoint_scaling_correct_revers_condition

endpoint scaling: invert condition
fix densities for new parser in BlackoilPvtProperties

that is one of the more subtle differences between the old and the
new parsers. now, valgrind does not seem to complain anymore, so everything
should be All Right (TM) ;)
Merge pull request #527 from andlaus/fix_blackoil_pvt_props_in_new_parser

fix densities for new parser in BlackoilPvtProperties