22.0 extend interface InDetIdentifier

Remove in-class static member giving value for invalid Identifier hash; now reply simply on the IdentifierHash::is_valid method. Rationalise the existing methods by factoring common code into new private member functions or a lambda. Provide more intuitive methods for get_next_in_phi etc (the module neighbours) which return the identifier in return value (as opposed to passing a reference argument). Update tests to include new methods.