HomotopyMixin: fix overwriting of seed
It is necessary to overwrite the seed within the HomotopyMixin class whenever the results of the latest optimization run are stored here. Note that that a necessary requirement for this is that theta must be greater than zero. Moreover, there are only two situations in which we need to overwrite the seed: when the class GoalProgrammingMixin is not used, or whenever it is the first run of the goal programming loop.