Value Translator should support ClassicalB and EventB parser

Currently the Value Translator only uses the ClassicalB parser. This leads to problems in translating values of Event-B machines. Especially translating enumerated set values like "front", "right" and "left" which are keywords in Classical-B, however not in Event-B throw exceptions. The Value Translator should be flexible, i.e. if an Event-B machine is loaded, the Event-B parser should be used and if an Classical-B machine is loaded, the Classical-B parser should be used.