Added get_wsi_at_mpp tests; fixed a few bugs

Author: NikolasSchmitz

monai/data/wsi_reader.py

@@ -1316,7 +1316,7 @@ def get_mpp(self, wsi, level: int) -> tuple[float, float]:
             # Here, x and y resolutions are rational numbers so each of them is represented by a tuple.
             yres = wsi.pages[level].tags["YResolution"].value
             xres = wsi.pages[level].tags["XResolution"].value
-            if xres[0] & yres[0]:
+            if xres[0] and yres[0]:
                 return convert_to_micron(yres[1] / yres[0]), convert_to_micron(xres[1] / xres[0])
             else:
                 raise ValueError("The `XResolution` and/or `YResolution` property of the image is zero, "