Add option to automatically determine the overscan axis

If the overscan axis is explicitly set to `None` in
`subtract_overscan`, the overscan direction is determined from the
shape of the overscan section, set the shortest dimension. (In case of
a square overscan, the default of 1 is picked.)

This can be convenient in cases where `fits_section` is used with an
(unknown) input string (for example, from an image's BIASSEC header
keyword): the user can let the function determine the appropriate
axis.

ccdproc/core.py

@@ -205,6 +205,9 @@ def subtract_overscan(ccd, overscan=None, overscan_axis=1, fits_section=None,
     if fits_section is not None:
         overscan = ccd[slice_from_string(fits_section, fits_convention=True)]
 
+    if overscan_axis is None:
+        overscan_axis = 1 if overscan.shape[0] > overscan.shape[1] else 0
+
     if median:
         oscan = np.median(overscan.data, axis=overscan_axis)
     else: