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 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. To detect this, and to make it clear that we internally rely on this variable, we rename __first_run to _gp_first_run.