Implicit casting to float value in predicates

NSPredicate does not support float values in query strings; only double is. If a value can be casted to a float, and a class has a float property, we can help the user by casting for her but it might lead to loss of precision.

@alazier @jpsim @astigsen 
Shouldn't we test edge cases of MAX_FLT and MIN_FLT?