Updates in data conversion

[constantinpape]

Some updates are needed to correctly support the data conversion.

1. Don't write calibration transform, this causes issues with the BigStitcher error computation during stitching (values are not in pixels, which is a bit confusing) and more importantly leads to rescaling in the image fusion. The latter point is a likely explanation for some of the issues we have with segmentation due to changes in data scale - double check this after the fix. The fix should be easy by just writing the voxel size information and not the calibration transform.
2. Write a MissingViews key initialized with an empty list (see below).
3. Support tiles in raw format in addition to tif (wait for example data)

[constantinpape]

Implemented by #10