Allow number of wells to increase during simulation

This has been tested with the SPE9 case with no problems detected, the minor cost of copying does not reduce performance (the speedup of #179 is much more significant). The codes assumes:
 - The number of wells can only increase during simulation.
 - The wells are always in the same (WELSPECS) order, so the N wells of the previous step will always be the first N wells of the current step.

Both assumptions should be correct with the current opm-parser.

This code has not been tested yet with actually increasing well numbers! But I propose it for merging anyway, since it does no harm. Also, we would need to merge OPM/opm-core#625 to properly test it, and that must be reviewed first.