Cooperating plans should be similar plans with same production unit - implement restriction or warning

[sloschert]

It is important that cooperating plans are plans that produce similar products with the same production unit.

Production unit is currently implemented (intentionally) as a free text field/string, which makes it difficult to check this requirement.

But we could implement a warning for a coordinator of a cooperation, if he/she wants to accept a cooperation request and when the production unit strings differ from one ore more already cooperating plans.

Another option would be that coordinators define an obligatory production unit when creating the cooperation.