Make create_staggered_kernel work with OpenMP

See merge request pycodegen/pystencils!123

Add DataHandling.add_arrays

See merge request pycodegen/pystencils!120

Automatic Finite Volume and Volume-of-Fluid discretization

See merge request pycodegen/pystencils!121

This adds a function for lazy people who want to write

```python
dh = create_data_handling((20, 30))
dh.add_arrays('x, y(9), z')
```

instead of

```python
dh = create_data_handling((20, 30))
dh.add_array('x')
dh.add_array('y', values_per_cell=9)
dh.add_array('z')
```

because in most cases you want to use more than one array.

Opencl datahandling

See merge request pycodegen/pystencils!119

this to your children or students)

PointerType should be rendered differently in OpenCL backend

Allow functions for Field.coordinate_transform

See merge request pycodegen/pystencils!118

Fix AssignmentCollection.{free_symbols,bound_symbols,defined_symbols} for non-Assignments

See merge request pycodegen/pystencils!114

   Add a star to correctly get atoms of InterpolatorAccess   

See merge request pycodegen/pystencils!115

FiniteDifferenceStaggeredStencilDerivation must be applied to field access

See merge request pycodegen/pystencils!109

Update AssignmentCollection.__repr__

See merge request pycodegen/pystencils!110

`Assignment Collection for y[0,0,0]` is usually not very helpful.

New representation:

```
In [6]:     forward_assignments = pystencils.AssignmentCollection({
   ...:         z[0, 0]: x[0, 0] * sympy.log(x[0, 0] * y[0, 0]),
   ...:     y[0,0] : x[1,1] +1
   ...:             })

In [7]: forward_assignments
Out[7]: AssignmentCollection: z_C, y_C <- f(x_C, x_NE)
```

otherwise the index gets lost

AssignmentCollection.__bool__

See merge request pycodegen/pystencils!108

Allow `CUSTOM_FIELD`s to have a different size in transformations.py

See merge request pycodegen/pystencils!107