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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

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`

-> different fields need different shape/stride symbols

- stencil plotting & transformation now in ps.stencil
- additional documentation & notebooks

Nils Kohl authored Mar 27, 2019 and Martin Bauer committed Apr 26, 2019

- introduced AbstractField and AbstractAccess

Fixes #28

This restructuring allows for easier separation of modules into
separate repositories later. Also, now pip install with repo url can be
used.

The setup.py files have also been updated to correctly reference each
other. Module versions are not extracted from git state

- all branches are added up

boundary conditions can specify how the index list should be built:
- list coordinates of domain or boundary cells (previous always inner)
- list all links or only the first link

- removed warnings from generated code
- made generated code string deterministic, generating the same twice
  gives binary equally files now

- was not used consistently before
- symbol names are expected to be valid C identifiers
- for complicated field names, the latex_name of field should be used

- small (length < 5) arrays with shape and stride information had to be
  memcpy'd to the GPU before every kernel call
- instead of passing the information as arrays, the single elements are
  passed
- leads to more function arguments, but simplifies GPU kernel calls

-> changes in all backends required

- field access are now equal if their underlying fields compare as equal
  not only if the id of their fields are equal

- if a field access is printed in compass or index notation seems to
  depend on the sympy version
- removed doctests that rely on the printed form

- added isotropic version

- new functions to easily extract stencil coefficients and visualize them
- Moved stencil functions from lbmpy to pystencils

- hash function was not correctly implemented
- sometimes this caused a bug where latex version was not correctly displayed

- better latex display for indirect accesses
- new field type 'custom': only custom fields can be accessed indirectly
  no static bounds check possible for custom fields

- different approach in pystencils: absolute indexing