Allow timestep index to be overridden

The Eclipse timestep index can not always be retrieved from the
SimulatorTimer object; the initial state is 0 in Eclipse (and does not
have an index in OPM), and the timestep in OPM is not increased until
*after* the timestep is completed (i.e. it is not initialized to -1).
Factor out code that writes solution field variables

We will reuse this later for also writing the initial solution.
Propagate initial state to the writer as well

The writeTimeStep method is called *after* each timestep and does
not include the initial state of the reservoir. If the writer wants
to dump the initial state of the reservoir, this must be done in
writeInit, which is called before the simulator is run, but after
the initial state has been set up.
Write initial solution when the writer is initialized
Introduce conversion function to avoid explicit copy

We already copy the data once from the OPM state into the ERT-managed
memory, so we may as well fudge a conversion routine in there (at the
expense of having to do this everywhere).
Convert permeability to milli-Darcy when writing
Don't write initial state in main timestep loop

The call to writeInit should take care of this instead.
Report after the timestep really is completed

If we report before we increment the timer, then we will report at
a time which is at the beginning and not at the end of the timestep.
Replace unclear constant with better documentation

The step number is zero before the first timestep has been taken, and
one after. The step number is one before the second timestep has been
taken, and two after. This was not clear from the text.
Do without the timestep index adjustment

If the output routines are called at the right place in the simulator,
then the numbering scheme coincide with that of Eclipse, and no
adjustments are necessary.
Provide routine to return the step length taken

The output routine needs to know which step that has been taken in the
past (to arrive at this result), not which step to take next going
forward.
Use the previous step length to calculate historic rate