Operator sites

"Operator" has been added as a new part type. It has sub-families called "Activator Binding Site" and "Repressor Binding Site". An Inducible Promoter is defined as a combination of an Activator Binding Site and a Promoter part. A Repressible Promoter is defined as a combination of Repressor Binding Site and Promoter. Only one transcription factor is allowed to bind to a single operator site -- this makes modeling much more convenient, but it also makes sense physically.