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).
It's not an identity search, sure. Could be `COMP_LIKE`. But then the code that wants to use this needs to know which handler supports which comparator. We want to start simple. And what you see here is just my definition of an `COMP_EQUAL` comparison for quantities.