CMakeLists: replace src-list with GLOB expression
python package structure and corresponding changes to CMakeLists.txt
- the content of python/crpropa is copied to site-packages on make install
- cpropa.py, crpropa.so are copied to site-pagackes/python

Usage:
- __init__.py includes a "from crpropa import *", keeping the current behaviour
- other python modules can be specifically included with "from python import XYZ"