Check exception of old numpy versions

I'm not able to install old numpy versions locally and the exception
documentation of numpy is non existent and I can't bother to go through
its source code, so I'll just see what the actions show.

src/gstools/field/pgs.py

@@ -70,16 +70,7 @@ def calc_pgs(self):
         pgs : :class:`numpy.ndarray`
             the plurigaussian field
         """
-        try:
-            mapping = np.stack(self._Zs, axis=1)
-        except np.exceptions.AxisError:
-            # if dim==1, `fields` is prob. a raw field & not a 1-tuple or
-            # equivalent
-            if self._dim == 1:
-                self._Zs = [self._Zs]
-                mapping = np.stack(self._Zs, axis=1)
-            else:
-                raise
+        mapping = np.stack(self._Zs, axis=1)
         pos_l = []
         for d in range(self._dim):
             pos_l.append(