deb: remove split of normal and -dbg packages

simpler, less prone to problems during build and easier to maintain
deb packaging: refactoring for py2/3 transition

- separate tests/data and tests/images files into shared package
  python-obspy-dbg for python-obspy and python3-obspy, also avoids
  lintian complaining about image files in /usr and makes a slim install
  without test data files possible
- move .py test files back into usual obspy packages
- rename executables for python3 obspy to e.g. obspy3-plot