Warn user when no file path is given in 'Add Operation Step'

[AliceBalfanz]

When configuring a operation on a dataset, there is the possibility to select the file path (e.g. when plotting a hovmöller diagram).
When selecting the path via the selector I can select my workspace directory, and then click on 'save file' and no warning is given, that I have only selected a directory and not a file or given a file name. The user should be warned to provide a file name.

[forman]

Closing this in favor of #102