Use array_type fixture; fix bugs for masked array inputs to functions

* Use `array_type` as needed `test_basic.py`
* Fix bug in `wind_direction()` for masked arrays when calm. Boolean
  indexing of a masked array resulted in modification of the mask
  itself.
* Fix bug in `smooth_gaussian()` where function returned a
  `numpy.ndarray` when given a `numpy.ma.array`
* Remove `test_direction_masked()`

src/metpy/calc/basic.py

@@ -104,8 +104,14 @@ def wind_direction(u, v, convention='from'):
         wdir[mask] += units.Quantity(360., 'deg')
     # avoid unintended modification of `pint.Quantity` by direct use of magnitude
     calm_mask = (np.asarray(u.magnitude) == 0.) & (np.asarray(v.magnitude) == 0.)
+
+    if hasattr(wdir, "mask"):
+        array_mask = wdir.mask
+    else:
+        array_mask = False
+
     # np.any check required for legacy numpy which treats 0-d False boolean index as zero
-    if np.any(calm_mask):
+    if np.any(calm_mask & np.logical_not(array_mask)):
         wdir[calm_mask] = units.Quantity(0., 'deg')
     return wdir.reshape(origshape).to('degrees')
 
@@ -798,8 +804,12 @@ def smooth_gaussian(scalar_grid, n):
     # Assume the last two axes represent the horizontal directions
     sgma_seq = [sgma if i > num_ax - 3 else 0 for i in range(num_ax)]
 
-    # Compute smoothed field
-    return gaussian_filter(scalar_grid, sgma_seq, truncate=2 * np.sqrt(2))
+    filter_args = {'sigma': sgma_seq, 'truncate': 2 * np.sqrt(2)}
+    if hasattr(scalar_grid, 'mask'):
+        smoothed = gaussian_filter(scalar_grid.data, **filter_args)
+        return np.ma.array(smoothed, mask=scalar_grid.mask)
+    else:
+        return gaussian_filter(scalar_grid, **filter_args)
 
 
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