setup.py not installing codegen

[JaroslavHron]

doing direct installation into prefix directory
python3 -s setup.py --no-user-cfg install --prefix=/usr/local/pkg/pyop2
will not install the codegen subpackage

it has to be explicitly listed as

setup( ..., packages=['pyop2','pyop2.codegen'], ....)

or with setuptools you coud use find_packages()

from setuptools import setup, find_packages

setup( ..., packages=find_packages(), ....)

[wence-]

Merged your pr. thanks.