Added RNA-Seq edgeR DGE workflow example
- added RnaSeqDgeExample and RNA-Seq-edgeR-DGE.ga: an example of the RNA-Seq edgeR DGE workflow (in progress).
- Constants: added constants for the SURFsara CTMM TraIT Galaxy server and for the RNA-Seq edgeR DGE workflow.
- GalaxyWorkflowEngine: added todo for separate check and/or better error message when the server is not available, working on appropriate extensions for downloaded files (in progress).