BUG: continuation of updateMethods with empty activeDesignVariables (#1540)

When activeDesignVariables are not set explicitly, all design variables are treated as active. These were allocated properly when starting from 0 but not when starting from an intermediate optimisation cycle (e.g. running 5 optimisation cycles, stopping and restarting).

TUT: added a new tutorial including the restart of an optimisation run to help identify future regression