Move the generic copy method to OperatorMixin
Move the generic copy method to OperatorMixin

The arithmetic operations required a copy() method, so a simple
implementation of this (just calling copy.copy or copy.deepcopy) was
moved from MultiObjective to OperatorMixin. More sophisticated objects
that have cached methods (like Misfit) will need to overload this.

victortxa/fatiando,santis19/fatiando,mtb-za/fatiando,cmeessen/fatiando,fatiando/fatiando,drandykass/fatiando,mtb-za/fatiando,rafaelmds/fatiando,drandykass/fatiando,cmeessen/fatiando,santis19/fatiando,fatiando/fatiando,victortxa/fatiando,rafaelmds/fatiando