- since the default value for Position is Nowhere now.
Also, remove reduntant header and change stream to ostream in to_strings

The concept has been reworked a bit.

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

It was trying to parse a position instead.

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

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.

Added parentheses for all "general" operator rules (string_operator, bool_operator, symset_operator and position_operator).

'unify the names of the rules').