Added filter step to the cartesian product campaign.

[RobbeDGreef]

This filter step can be used to remove any variable combinations which you do not want to use.

def filter_unwanted(variables):
    return True
    
CampaignCartesianProduct(
    ...
    filter_func=filter_unwanted
)

[apaolillo]

that's a great idea! Could you add a small example of your filter in tests/campaigns/? You can use the examples there as inspiration/base.