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

For sympy, oo s a number. So pystencils prints a type infinity a double
INFINITY as INFINITY.0

There is now a superior implementation in pystencils_autodiff.

This is actually a AST Node for wrapper code. pystencils_autodiff has
now a AST printer for framework integration code. So framework AST nodes
are now there and kernel AST nodes remain here.

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
()

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.

- 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