LEDGE-242: get rid of ambiguity in the parser by stashing ASTvalue in a
field of common ancestor class of all relation predicate nodes.
LEDGE-242: reworked toString implementation to account for the fact that
ASTvalue child of RelationPredicate nodes was moved to a local field
LEDGE-242: implemented exixtence predicate
LEDGE-242: return a context wrapping a MissingNode when no child node
matches a predicate while [<predicate>] evaluation to avoid NPE in
existencePredicate
LEDGE-243: added integer comparison operators if the value is not
numeric match is considered to be false.
LEDGE-243: add an error message if a field used for integer comparison
contains a non-numeric value.