It's sometimes useful to have nonmember operators in c++
e.g. C operator+(int, const C&)
(So int can come first)

These could be defined outside the "cdef cppclass" section
but if they were in a pxd file they weren't easy to import
and use.

They can now be defined with the "cdef cppclass" and will
be found and used appropriately
e.g.

cdef cppclass C:
  C operator+(int, const C&)

Appropriate tests have been added