Rerun notebook before editing it.
Add an example of Context.register.
Add mpi-only example.
Add an mpi-only example.
Add and edit some comments and text.
Tweaks to features.ipynb
Fix error in an output cell.
Merge pull request #559 from enthought/feature/update-features-notebook

Update features notebook
Merge branch 'master' into improve-examples
Fixes context.register to allow registration of compiled funcs.