Checks wether Position is_inside, is_outside, or equals to other Position given the SentenceContext.

Adding some methods to values for use by operators, e.g. equals and is_subset_of/intersection. Some are just stubs.

Add PositionRef value class, add Position:: nowhere/begin/end, add SentenceContext get_[abs|rel]_position takint Position or Positionref
Add tests for sc's get_*_position