have comparisons return bool when both objects are values

[ekorman]

Currently, when two Equatable objects (that are not symbolic but have values) are compared, the result is a Boolean object instead of an actual bool. this causes some undesired behavior due to example bool(Boolean(False)) == True. and indeed test_add_prediction and test_create_image_dataset_with_detections were bad tests and actually should have been failing (see their updates)

This PR updates the following methods of Equatable to return bools when both sides have values:

• __eq__
• __ne__
• in_
• __gt__
• __ge__
• __lt__
• __le__