fix mongoengine and peewee exceptions when filtering for non-integer values

Currently, peewee and mongoengine will throw an exception when you enter anything that isn't an integer into an Integer Filter. This pull request fixes the issue by adding a new converter for float types and validation to both integer and float types. Tests are also added for the new float type.

This commit also consolidates most logic for date, datetime, and time BETWEEN filters into model/filters.py.