magnification issue

[Karenxzr]

Added magnification argument for tile extracting. Default setting is magnification=None, then previous version that used level for tiling will be ran. Whenever a valid magnification is set, level is ignored and automatically be calculated for openslide backend.

Example: region0 = wsi.slide.extract_region(location = (20000, 20000), size = (256, 256),magnification='2.5x') region1 = wsi.slide.extract_region(location = (20000, 20000), size = (256, 256),magnification='5x')

[jacob-rosenthal]

The problem with the tests hanging should be fixed (#196) so merging dev into your branch should hopefully make the tests pass