Adding a few safety features

[stevet40]

This fixes #14 and fixes #18.

I have printed a warning in sample_ode_from_base and solve_odes_forward to catch and alert a user if a negative atol or rtol (or a tolerance looser than 1e-2) is passed to odeint. I have also changed the no_sigma behaviour so this is no longer an explicit setting handled by the user. Rather, a VESDE will by default invoke no_sigma=False for the ScoreModel class, and anything else will default to no_sigma=True. I think these are the optimal choices, and they could be manually overridden if necessary by editing the class attribute.

Separately to those two issues, I also adjusted the default odeint solver options dictionary, to set min_step to 1e-6 as standard. We have historically always done this in our own applications, and it makes the solver more robust against dt underflow errors.