Add quantity data value handler

Similar assumptions as in TimeHandler (see #49):
* The equality field is a simple, human readable string representation.
* Ignore lower/upper bounds when searching.
* Ignore decimal places beyond PHP's build-in "14 significant digits" string conversion.
* StringHasher is used to be able to have a maximum length. Otherwise it would be unlimited since the unit doesn't have a limit.
* Order by float value. Ordering by unit doesn't make any sense.
* No unit conversion (for now, may be implemented in a future version).
Splitting this does not really solve the problem outlined here https://github.com/wmde/WikibaseQueryEngine/pull/54#discussion_r13647102