It is more intuitive that way, e.g. equal(2,3) is never true, equal(3,3) is always true.
Nowhere is only equal to Nowhere, and Begin or End are equal to nonspecial positions
depending on current position in the sentence context.

* reset_values function to set all Values to their defaul-construct state
* read-only access to all variables of some type via get_all<T>
* Variables cloning
* convenience functions: get_or_throw, get_value
Value subtypes now contain a value_type typedef and require a const value_type& get_value() const member function.

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