[Core] Allow query differences

[yuripourre]

"Why" requires a more detailed explanation than yes or no. But it could be done for sake of simplicity.

Action walk = new Action();
walk.is(move);

Action run = new Action ();
run.is(walk);
run.set("pace", new Concept ("fast"));

Map<String, Pair<Concept, Concept>> result = why(run).isDifferentThan(walk);

Related to #79

[yuripourre]

This can be very useful to determine what and/or when something changed in an instance, like: "started to rain" or "became the king of France".

[yuripourre]

This can be very useful to find insights, like the only mammal that flies or the youngest person in a family.