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.

Conflicts:
	libwccl/CMakeLists.txt

Fixing expected interface of sets i.e. bool intersects function, not a function returning actual intersection

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

Conflicts:
	libwccl/CMakeLists.txt

add a VariableAccesor class in Variables and corresponding create_accesor and get_fast functions in Variables, plus tests

LogicalPredicate, abstract base class for predicates that are logical functions taking any number of arguments