Add own implementation for printing an unknown function

Using the SymPy implementation for printing a sympy.Function has some issues:

• you cannot use cast_func as an argument -> SymPy prints `cast_func(2,float)
• typed numbers are not typed any more -> e.g. pow(x, 0.25) instead of pow(x, 0.25f)

Maybe a disadvantage or an advantage:

• arbitrary Functions: sympy.Function('my_function') are now allowed.

Would have prevented commits like:

Fix: type of sqrt(int) was int not floating point type

pystencils_tests/test_cuda_known_functions.py

@@ -32,8 +32,7 @@ def test_cuda_but_not_c():
     })
 
     ast = pystencils.create_kernel(assignments, 'cpu')
-    code = str(pystencils.show_code(ast))
-    assert "Not supported" in code
+    print(pystencils.show_code(ast))
 
 
 def test_cuda_unknown():
@@ -46,4 +45,3 @@ def test_cuda_unknown():
     ast = pystencils.create_kernel(assignments, 'gpu')
     code = str(pystencils.show_code(ast))
     print(code)
-    assert "Not supported in CUDA" in code