Allow complex numbers in odeint

[gautierronan]

This PR allows complex numbers in torchdyn.odeint. Only a few simple changes were needed.

Also moves the default save_at instantiation in odeint to avoid a systemic warning when using adaptative step solver.

[Zymrael]

Thanks for the PR! It'd be useful to also include a simple test with odeint applied to complex numbers.

[gautierronan]

I added a test function. Not sure how to integrate with the rest of the code however, I hope it's fine !

[Zymrael]

Looks good! Thanks again