Do not allow fields with identical names.

[MaxvdKolk]

Writing data fields with identical identifiers (the name string) breaks the files. On Paraview 5.5 this crashes on loading. Add a check that errors whenever we try to insert another field with the same name?

[MaxvdKolk]

The error message is now along the lines of "Array already contains field %s in fields %v". It seems that the xml name is not set explicitly, it comes through the name of the field in the struct. This is fine, but might limit the clarity of the error message? It might be nicer to append some additional info, where it goes wrong?

[MaxvdKolk]

The error message is now along the lines of "Array already contains field %s in fields %v". It seems that the xml name is not set explicitly, it comes through the name of the field in the struct. This is fine, but might limit the clarity of the error message? It might be nicer to append some additional info, where it goes wrong?

For now the behaviour seems fine. Directly after a call this error appears.