- 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`

```

Add global_declarations to cbackend

See merge request !5

Make subexpressions optional for constructing an AssignmentCollection

See merge request !3

Address of SymPy-Function `address_of`

See merge request !1

This enables astnodes.Nodes to have a member required_global_declarations
by which they can specify a global declaration required for their usage.

When introducing new people to pystencils it's often simpler not to
differentiate between `main_assignments` and `subexpressions` in the
beginning.
Also for simple kernels subexpressions are often not needed, since
intermediate symbols can also be set in main_assignments.

Subexpression should be kept for expert users.

- previous method did not work with kernels generated for walberla where
  block size changes are made at runtime
- device query does not always work, since the compile system may have
  no GPU or not the same GPU
-> max block size is passed as parameter and only optionally determined
   by a device query

-> different fields need different shape/stride symbols

Block.subs method tried to be too smart:
a = field[..]
b = a + b

was "simplified" incorrectly to
b = field[...] + b

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

- updates in readme text
- added release script

- typos, more content
- added numba to benchmark comparison