Installing (some) python examples

[KrisThielemans]

There starts to be some useful stuff in examples\python. Arguably, some of this should be installed in the stir module, e.g. stir.examples.plot_sinogram_profiles.py, or stirextra.plot_sinogram_profiles.py or even stir.stirextra.plot_sinogram_profiles.py.

Somewhat related is one of our oldest outstanding issues #5, which uses setup.py but the fix did move stirextra into stir, which as why i never merged that for backwards compatibility. Obviously, setup.py is now outdated and AFAIK superseded by pyproject.toml. This is all stuff I (want to) know next to nothing about though.