It is coherent now with the SentenceContext::is_inside(int abs_pos)
- SentenceContext::is_inside(Position pos) is added, while
Position::is_inside(SentenceContext) is removed

I.E. empty sentence shouldn't be passed anyway and that is what
Operator class is checking for. But otherwise, at the level of
operator, it can deal with a sentence that has current pos outside
of boundaries.

Note: still not implemented internals of agreement

Values cleanup and doxy: remove some redundancies, make the Begin/End constants in Position an enum, change tag_ref to contents

The agreement stuff isn't implemented yet.

It is called "flex" atm but it is subject to change.

Add the tagset-symbol-getter operator (GetSymbols), bump required Corpus2 version to 1.0.2 since new features are needed.