Use knime relative paths

[jpfeuffer]

I am preparing a PR that allows to use knime relative paths in the Input File and Input Directory node.
With knime://knime.workflow/... you can specify files relative to the workflow folder. This could be used to avoid the constant readding of the input files, at least for developers.
We could event think about using an FTP connection to get the data from a server.

[jpfeuffer]

See status of genericworkflownodes/GenericKnimeNodes#141