Add y-step parameter to reader

In order to read only the rows we want and do that efficiently.
Add UnsignedLong edf support

which is actually a 32bit uint
Use ROI height in reader as a relative mark

The reading is now consistent with region-of-interest, i.e.
the reader reads [y, y + roi-height].
Merge pull request #53 from ufo-kit/reader-improvements

Reader improvements