Reimplement create_staggered_kernel

Closes #16

See merge request !93

move_constants_before_loop needed to be changed to move already-existing variables up if necesesary and to preserve constness when renaming variables

Add __getnewargs__ to TypedImaginaryUnit

See merge request pycodegen/pystencils!94

Add own implementation for printing an unknown function

See merge request pycodegen/pystencils!96

also add a test for subexpressions

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)

There was a problem when pickling TypedImaginaryUnit since it was using the __getnewargs__ of TypedSymbol.

More staggered grid improvements

See merge request pycodegen/pystencils!92

It takes a list of assignments to staggered fields and otherwise behaves like create_kernel.
The old create_staggered_kernel was not removed because all the optimizations still need to be ported over.

When storing fluxes on a staggered grid, the usual sign convention is that
fluxes point outward from the cell. Previously, we did not respect that as
staggered_access("E") would return the same thing as staggered_access("W")
would when called from the eastern-next cell. Now, when a field is declared
as STAGGERED_FLUX, it returns an accessor with a prefactor of -1 in that
case. The previous behavior where sign is not reversed is still useful when
e.g. storing sums (e.g. mean values) instead of differenes (e.g. finite
difference fluxes) on the staggered grid.

NW (-1/2, 1/2) and the like were previously mapped to the wrong cell

Change `Field.__repr__` to match representation in `pystencils.fields`

See merge request !90

Avoid `hash()` in Field.hashable_contents

See merge request !91

cbackend: short-cut _print_Conditional if condition is a boolean atom

See merge request !89

fix minor regressions introduced with !86

See merge request !88

According to the documentation, 0 is W and 1 is S

hash should not be used in hashable_contents because even if we will use
a deterministic hash function we will end up non-deterministic since
hash is initialized with a random seed.

__repr__ of Field should look like

```
y: uint16_t[246,704,558]
x(3): float[20,40]
```

to match representation when using `pystencils.fields`

OpenCL macOS support

See merge request pycodegen/pystencils!87

Staggered field access and staggered fields with fluxes to edges/faces

See merge request pycodegen/pystencils!86