python: Fix error message in LB nodegrid

The np.array() casts the entire array to string if the last element
is a string, causing the assertion in LBFluidRoutines() to complain
about the first element being a string, which it is not in the user
input. The logic for the assertion in HydrodynamicInteraction() can
also be replaced by utils.check_type_or_throw_except().

src/python/espressomd/lb.pyx

@@ -82,11 +82,9 @@ cdef class HydrodynamicInteraction(Actor):
         return _construct, (self.__class__, self._params), None
 
     def __getitem__(self, key):
-        if isinstance(key, (tuple, list, np.ndarray)):
-            if len(key) == 3:
-                return LBFluidRoutines(np.array(key))
-        raise Exception(
-            f"{key} is not a valid key. Should be a point on the nodegrid e.g. lbf[0,0,0].")
+        utils.check_type_or_throw_except(
+            key, 3, int, "The index of an lb fluid node consists of three integers, e.g. lbf[0,0,0]")
+        return LBFluidRoutines(key)
 
     # validate the given parameters on actor initialization
     ####################################################