Extend testsuit

This MR extends the test cases of pystencils.

Other changes made in this MR:

1. Usage of correct backends for the codegen instead of C Backend for all
2. Deletion of unusable function.
3. Correction of CUDA and OpenCL Array handler

pystencils/fd/finitedifferences.py

@@ -121,29 +121,6 @@ class Discretization2ndOrder:
             new_args = [self._discretize_spatial(a) for a in e.args]
             return e.func(*new_args) if new_args else e
 
-    def _discretize_diff(self, e):
-        order = self._diff_order(e)
-        if order == 1:
-            fa = e.args[0]
-            index = e.target
-            return (fa.neighbor(index, 1) - fa.neighbor(index, -1)) / (2 * self.dx)
-        elif order == 2:
-            indices = sorted([e.target, e.args[0].target])
-            fa = e.args[0].args[0]
-            if indices[0] == indices[1] and all(i >= 0 for i in indices):
-                result = (-2 * fa + fa.neighbor(indices[0], -1) + fa.neighbor(indices[0], +1))
-            elif indices[0] == indices[1]:
-                result = 0
-                for d in range(fa.field.spatial_dimensions):
-                    result += (-2 * fa + fa.neighbor(d, -1) + fa.neighbor(d, +1))
-            else:
-                assert all(i >= 0 for i in indices)
-                offsets = [(1, 1), [-1, 1], [1, -1], [-1, -1]]
-                result = sum(o1 * o2 * fa.neighbor(indices[0], o1).neighbor(indices[1], o2) for o1, o2 in offsets) / 4
-            return result / (self.dx ** 2)
-        else:
-            raise NotImplementedError("Term contains derivatives of order > 2")
-
     def __call__(self, expr):
         if isinstance(expr, list):
             return [self(e) for e in expr]