Rework pikos run

This PR makes the pikos-run script more stable (especially) on focused operation

- Use importlib to retrive the functions on focused operations.
- Support retriving functions defined inside the executed script without having to give the script name. 
- Better behaviour when parsing the focused functions argument string
- Better warning message when built-in functions are provided for focused operation

**Pending**:

- [x] Tests for the runner
- [x] Update docstrings
- [x] Documentation