Download dataset project

We now support Download of Original Files and png/jpg/tiff for Projects & Datasets.
Since all download code has been refactored, need to do a full download test (sorry)!
For each Download, check you get all the files you expect.

To test, find various images:
 - OMERO 4.4.x images that have original archived files.
 - Images that come from a single file (E.g. tiff)
 - Images that come from multiple files E.g. DV + log file / lei etc.

- For each type of image, select a single image and Download originals.
- Also download jpg/png/tiff for any single image.
- Now select multiple images (any combination of the above) and download originals.
- Also download jpg/png/tiffs for multiple images.
- Select a single Dataset containing test images. Download original images.
- Also download jpg/png/tiff for Dataset
- Repeat for multiple Datasets.
- Select a single Project containing test images. Download original images.
- Also download jpg/png/tiff for Project
- Repeat for multiple Projects.