This commit creates an OPM-specific extension of the DOUBHEAD vector
of a restart step. We reuse the TSINIT item (zero-based index 1) to
store the next timestep. Local testing suggests that ECLIPSE does
not use this value as part of restarting a simulation so this item
is a reasonable compromise for creating a mostly ECLIPSE-compatible
restart file that still enables communicating the suggested next
timestep if we're restarting Flow from a result set created by Flow.
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