Tolerate very slightly non-square pixels (#123)

If pixel X and Y dimensions are different but within a 1e-4 relative tolerance,
issue a warning and use the X dimension when stitching and writing output file
metadata. Dimensions outside of that tolerance will still raise an exception.

Fixes #122

Author: jmuhlich

ashlar/reg.py

@@ -290,10 +290,15 @@ def pixel_size(self):
             else:
                 value = v_units.value(UNITS.MICROMETER).doubleValue()
             values.append(value)
+        values = tuple(values)
+        if not np.isclose(values[0], values[1], rtol=1e-4, atol=0):
+            raise Exception(f"Can't handle non-square pixels {values[::-1]}")
         if values[0] != values[1]:
-            raise Exception("Can't handle non-square pixels (%f, %f)"
-                            % tuple(values))
-        return values[0]
+            warn_data(
+                f"Pixel size is slightly non-square {values[::-1]}. Using"
+                f" {values[1]} for both dimensions."
+            )
+        return values[1]
 
     @property
     def pixel_dtype(self):