Update Sherpa_i to Sherpa 3.0

This comes with many changes to the Sherpa structure, user interface, and syntax, which are all reflected in the interface code, in the base fragments, in the examples, but not yet in the documentation everywhere. Will update the documentation continually over the next weeks, but want to get merge this early to allow it to go into a release for wider testing.

The simple Sherpa_i example runs successfully in local testing.

/cc @cgutscho