decide on how to handle nans

[brookehus]

Right now, we have a nan checker implemented in the geometry class, which raises a ValueError if a nan is found and is called in the following geometry methods:

• get_distances
• get_angles
• get_dihedrals

These methods are invoked during network operations which occur both in training and in simulating. In simulating, we may not want a straight out ValueError - we likely want the option to get some information so we can debug what's going on.