Address of SymPy-Function `address_of`

Some CUDA functions (like atomic_add) require pointers to data. This PR adds a SymPy function representing the C address-of operator (&).

I tried to trigger cse to show a problem related to this function (dummy variables were not typed correctly as pointer). I'll include the fix in a follow-up PR.

pystencils_tests/test_address_of.py

+
+"""
+Test of pystencils.data_types.address_of
+"""
+
+from pystencils.data_types import address_of, cast_func, PointerType
+import pystencils
+from pystencils.simp.simplifications import sympy_cse
+import sympy
+
+
+def test_address_of():
+    x, y = pystencils.fields('x,y: int64[2d]')
+    s = pystencils.TypedSymbol('s', PointerType('int64'))
+
+    assignments = pystencils.AssignmentCollection({
+        s: address_of(x[0, 0]),
+        y[0, 0]: cast_func(s, 'int64')
+    }, {})
+
+    ast = pystencils.create_kernel(assignments)
+    code = pystencils.show_code(ast)
+    print(code)
+
+    assignments = pystencils.AssignmentCollection({
+        y[0, 0]: cast_func(address_of(x[0, 0]), 'int64')
+    }, {})
+
+    ast = pystencils.create_kernel(assignments)
+    code = pystencils.show_code(ast)
+    print(code)
+
+
+def test_address_of_with_cse():
+    x, y = pystencils.fields('x,y: int64[2d]')
+    s = pystencils.TypedSymbol('s', PointerType('int64'))
+
+    assignments = pystencils.AssignmentCollection({
+        y[0, 0]: cast_func(address_of(x[0, 0]), 'int64'),
+        x[0, 0]: cast_func(address_of(x[0, 0]), 'int64') + 1
+    }, {})
+
+    ast = pystencils.create_kernel(assignments)
+    code = pystencils.show_code(ast)
+    assignments_cse = sympy_cse(assignments)
+
+    ast = pystencils.create_kernel(assignments_cse)
+    code = pystencils.show_code(ast)
+    print(code)
+
+
+def main():
+    test_address_of()
+    test_address_of_with_cse()
+
+
+if __name__ == '__main__':
+    main()