Negative Gaia parallaxes

[bfhealy]

Our features include Gaia parallaxes, some of which are negative. The negative sign broadly means that those parallax values have larger uncertainties than positive parallaxes (e.g. https://arxiv.org/abs/1804.09376). Since this is an important feature when combined with magnitudes for the ontological classes, we should investigate how the inclusion of negative parallaxes may influence classifications.