Remove floor, ceiling for integer symbols

See merge request pycodegen/pystencils!14

fix compiler options for macOS

See merge request pycodegen/pystencils!12

Stephan Seitz authored 5 years ago and Martin Bauer committed 5 years ago

- rename header Field.h is not a unique name in waLBerla context
- add PyStencilsField.h
- bindings were lacking data type

functions like absolute values could not be used in CUDA backend since
known_functions was replaced instead of updated

Add CudaBackend, CudaSympyPrinter to extract CUDA-specific code from CBackend, CustomSympyPrinter
Cuda built-ins are added to CudaSympyPrinter.known_functions to use them as sympy.Function

# Conflicts:
#   pystencils/cpu/cpujit.py

- version has to be obtained from file when no git repo is available
  like in tar-ball distributed by pypi

- backend, target and compile are now normal members of the
  KernelFunction node and populated in constructor

Destructuring field binding

See merge request !4

DestructuringBindingsForFieldClass defines all field-related variables
in its subordinated block.
However, it leaves a TypedSymbol of type 'Field' for each field
undefined.
By that trick we can generate kernels that accept structs as
kernelparameters.
Either to include a pystencils specific Field struct of the following
definition:

```cpp
template<DTYPE_T, DIMENSION>
struct Field
{
    DTYPE_T* data;
    std::array<DTYPE_T, DIMENSION> shape;
    std::array<DTYPE_T, DIMENSION> stride;
}

or to be able to destructure user defined types like `pybind11::array`,
`at::Tensor`, `tensorflow::Tensor`

```