Prevent parameter with duplicate name

[Matthias Schoettle]

Currently, it is possible to add a new parameter to an operation and give it a name, which already exists.

However, it is not possible to edit an existing parameter and give it a name that already exists. This is prevented already.

The same should happen when adding new parameters.

[Shihang Zhu]

Resolves #446: Prevent duplicate parameter name by adding a uniqueness check

→ <<cset 78d996cf3d89>>

[Shihang Zhu]

Merged in Issues/446-duplicate-name (pull request #82)

Resolves #446: Prevent duplicate parameter name by adding a uniqueness check

→ <<cset 4d0a4c1351d0>>