round downsamples ?!?

[ap--]

I believe the way openslide calculates downsamples is not correct. I assume that there is a reason why the downsample factor in openslide is calculated as seen below. But my guess is that this is incorrect and should actually be rounded to whole numbers.

downsamples:
https://github.com/bayer-science-for-a-better-life/tiffslide/blob/ff2b18c0c067cd9a29f38fe6abe6f9dddf59cd5d/tiffslide/tiffslide.py#L231-L234

lower level offsets:
https://github.com/bayer-science-for-a-better-life/tiffslide/blob/ff2b18c0c067cd9a29f38fe6abe6f9dddf59cd5d/tiffslide/tiffslide.py#L434-L438

Other References:

• Inconsistent pixel value when fetching with non zero offset openslide/openslide#417
• Compatibility issue with read_region  #51

TODO:

• try to find examples of whole slide images that DO NOT have (almost) integer scaling.
• try to figure out why openslide calculates downsampling as it does
• implement a switch in tiffslide that rounds downsamples and switches to (more correct?) integer scaling between levels
• create a test script using openslide for finding examples that would contradict my intuition and ask image.sc users to run this on as many images as possible.