enh: improve sanity checks for BackgroundRollMed

CHANGELOG

@@ -1,3 +1,5 @@
+0.18.1
+ - enh: improve sanity checks for `BackgroundRollMed`
 0.18.0
  - BREAKING CHANGE: mask postprocessing did a morphological opening instead
    of a morphological closing, failing to remove spurious noise

src/dcnum/feat/feat_background/bg_roll_median.py

@@ -119,7 +119,7 @@ def check_user_kwargs(*,
         """Check user-defined properties of this class
 
         This method primarily exists so that the CLI knows which
-        keyword arguements can be passed to this class.
+        keyword arguments can be passed to this class.
 
         Parameters
         ----------
@@ -132,7 +132,8 @@ def check_user_kwargs(*,
             `kernel_size` will not increase computation speed. Larger
             values lead to a higher memory consumption.
         """
-        assert kernel_size > 0
+        assert kernel_size > 0, "kernel size must be positive number"
+        assert kernel_size % 2 == 0, "kernel size must be even number"
         assert batch_size > kernel_size
 
     def get_slices_for_batch(self, batch_index=0):