Fix get_type_of_expression for constants like sympy.pi

See merge request pycodegen/pystencils!35

Problem some constant expressions are neither Float,Integer,Rational but
don't have arguments.

>>> isinstance(pi, Integer)
False
>>> isinstance(pi, Float)
False
>>> isinstance(pi, Rational)
False
>>> pi.args
()

Implement sp.Sum, sp.Product

See merge request !38

Contest: ignore two more files if waLBerla is not available

See merge request pycodegen/pystencils!40

The line is only need when running pytest form the root directory

Pre-push hook

See merge request !36

Remove main methods from tests (sorry for adding them)

See merge request !37

Conftest: waLBerla, kerncraft

See merge request !39

Sum and Product have a indexing variable which is a Atom but not a free
symbol

... or code will be executed when pytest is collecting the tests.
I found out that I can use "-s" to convince vim-test to show me test
output.

- how could this not have been caught by the tests?!

- str(e) did work in previous pytest version
- but the correct usage is str(e.value)
- in newer pytest version the string for the e object is changed so
  the tests did not work any more

- numpy constants get directly their numpy type
- integer functions check for integer types at construction

- for example: sqrt(cast_func(3, double)) previously printed "sqrt(3)"
  instead of "sqrt(3.0)" leading to errors in CUDA code

Add KernelFunction.fields_written

See merge request !33

AES-NI Random Number Generator

See merge request !30

Fix error message of CBackend for unsupported nodes

See merge request !27

Bugfix: Readd __launch_bounds__ for dialect 'cuda'

See merge request !32

Bugfix: TypedSymbol.is_negative should not be implemented in terms of super().is_positive

See merge request !31

super().is_positive

This can lead to surprising simplifications

Philox tests and clean up

See merge request !28

- allow AST nodes in assignment collection to e.g. put RNG nodes in
  LB method