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

Nils Kohl authored 5 years ago and Martin Bauer committed 5 years ago

- introduced AbstractField and AbstractAccess

Fixes #28

When two loops have assignments to the same symbol with different
rhs and both are pulled before the loops, one of them is now renamed.
Previously one of them was left inside the loop.

Fixes #27