Add QuantityHandler

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).
I have no idea why this is an array but in this case I don't care. The other way around would be worse (as it happened with the `ORDER BY` fields names not being an array).