Fix get_type_of_expression for constants like sympy.pi

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

>>> from sympy import *
>>> isinstance(pi, Integer)
False
>>> isinstance(pi, Float)
False
>>> isinstance(pi, Rational)
False
>>> pi.args
()

pystencils/data_types.py

@@ -378,7 +378,7 @@ def collate_types(types):
 
 
 @memorycache(maxsize=2048)
-def get_type_of_expression(expr, default_float_type='double', default_int_type='int64'):
+def get_type_of_expression(expr, default_float_type='double', default_int_type='int'):
     from pystencils.astnodes import ResolvedFieldAccess
     from pystencils.cpu.vectorization import vec_all, vec_any