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