Introducing base abstract class FunctionalOperator and template Operator<T> for a functional operator returning given type of Value. Assortment of copy methods and apply methods.

Intended as a base class for library-user-level operators and rules. Allows manipulation of Variables via get, set. Intoduces cloning support.

Doesn't hurt and can help.

Some other refactoring too, some augmented comments etc.

This is to accomodate rules in future, and repurpose "Operator" to be a library api-level class for users of the library, for parsed functional operators.

It's a change from if x then y else z, which wasn't exactly proper anyway without parentheses.

RelativePosition operator, represented by "+" in WCCL (at least initially). Returns Nowhere for Nowhere, or a Position shifted by given offset to the position given (Begin and End are first translated into normal Positions).

The Equals<T> now uses helper class EqualityComparer<T> and distinction is in the helper class specialisation.

It's the "setvar($V, blabla)" operator that sets $V to blabla and always returns True.

Somewhat nicer to use and guards against static init order fiasco.

It is used wherever in WCCL variable is seen in read-only context, i.e. outside of operators that are setting its value like setvar.

Generally we want to avoid accidental change of value of a constant or of a variable. Originally I'd copy value on access, but this changed it so a const Value is returned instead and copy is done only when it's needed.

Functions operate on it now instead of on just SentenceContext. It's open to additions in future. Main purpose atm is to hold Variables as well as SentenceContext.
Done some cleanup in code too. Updated tests to reflect the FunExecContext change as well.

Making the default for "value-if-false" parameter to be a static, initialized on demand, member of Conditional.

Conditional operators realised as if..then..else expression and operator "? val ? pred" with some tests for StrSet specialization

Adding specialization of Equals for positions, as they logically need to take into account actual sentence context for full comparison.