Added lrel support to Tutor.
Added documentation for slicing.

All aspects of slicing have now been documented:
- Slicing without and with step for list, string, and node.
- Slicing assignment without and with step for list and string.

The single (overcrowded) concept page for assignment has been split into
8 subconcepts describing each flavor of assignment.
Merge branch 'master' of https://github.com/cwi-swat/rascal.git
Implemented,tested and documented slicing assignment on nodes