Update features notebook

[bgrant]

1. Add an example of Context.register
2. Add an example of running with MPI-only
3. Add and edit some comments and explanatory text

[kwmsmith]

@bgrant made some small tweaks, mostly just removing the version tags for different features. Looks great, thanks for updating.

[bgrant]

Works for me. I reran the notebook because there was an error in one of the output cells.