Add keep_going keyword argument to IRC

[ehermes]

This PR does a couple of things:

1. Adds a keep_going keyword argument to IRC. When keep_going=True, if an IRC inner iteration fails, the IRC optimization will print a warning that the trajectory is no longer accurate, but it will continue with the IRC. This helps when people are using IRC as a way of finding connected minima, but they do not care about the entire IRC trajectory.
2. Fixes a bug where creating an IRC instance monkey patches the get_W method of the base PES class, resulting in all subsequent optimizations using that same get_W method.
3. Minor code cleanup and type hinting.

[Andrew-S-Rosen]

Huge thank you for this!!!