Add the option to choose the instance_identifier

[henninggaertner]

Issue description

The user should be able to choose a name for the instance of the step, as to differentiate another one with the same method. This should be done with a field called "name step" or something along those lines, can be pretty much copied but needs to be adapted from the old views.py and urls.py

Acceptance Criteria

• [ ] the field is shown
• [ ] on default the field is empty, indicating the default will be used (backend handles this)
• [ ] if the user fills in something, the instance_identifier should be set in the name_output or however it is called

[henninggaertner]

All necessary base methods are there, now just needs to be implemented in the MethodForms in combination with the Method class itself