More staggered grid improvements

See merge request !92

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 !87

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

See merge request !86

Fix create_cuda_kernel to allow CUSTOM_FIELDS to have a different size

See merge request !83

Remove duplicated uint typedef

See merge request !84

Add Field.itemsize (yields Field.dtype.numpy_dtype.itemsize)

See merge request !82

Oops, forgot a return in TextureCachedField.reproducible_hash

See merge request !81

TextureCachedField was using `hash(...)` to disambiguate its instances.
However, `hash` is randomized and will hinder reproducible code
generation

Use reproducible hashlib for representing TextureCachedField

See merge request !80

Add sympy to known_third_party in .isort.cfg

See merge request !79

I have installed the newest master of SymPy as editable package.
So `isort` thinks it's one of our libraries.
With this config SymPy is declared as a third party dependency.

Support complex numbers

See merge request pycodegen/pystencils!72

Bugfix: textures should be implemented as textures without changes

See merge request pycodegen/pystencils!73

Add __str__ representation for TextureDeclaration

See merge request !74

Add option to omit globals when printing C code

See merge request !75

Run opencl without pycuda

Closes #15

See merge request !77

correctly print RNG nodes

See merge request !78