Implement possibility to not set a parameter

[jpfeuffer]

Some tools have conflicting parameters (which they shouldnt for their default values, although many tools dont specify defaults) and since GKN always adds them, it comes to errors.
We could accept empty strings (mind that we have to deactivate integer checking for empty strings then) and if the parameter is empty, it is not added.

[jpfeuffer]

And we have to allow the empty selection in the restricted strings. This maybe has to added in a CTD for such a tool.