Remove most unicode from public API

[kbarros]

Replace keyword arguments:

• Δt with dt
• λ with damping

Using the old symbols will emit a deprecation warning, and will break in 0.6.

[ddahlbom]

Took a quick pass over this. Looks good to me.

You want to eliminate the omegas and and energy_epsilon at a later point?

[kbarros]

Follow-up from discussion on Slack:

For LSWT, I will replace energy_ϵ with a new maxshift that only applies the shift if the quadratic hamiltonian has negative eigenvalues, and then, only as much as needed (up to maxshift). Then additionally, I will add energy_cutoff that cuts off intensities below an energy threshold, for consistency with KPM. Some of this might get pushed to 0.6.

For the ω related changes, I think we will do something like energies = ... where the right-hand side is a Julia Range. This new interface would take liberty to adapt the dt as needed. I'd like more time to think about it, and it might also get pushed to 0.6.

The immediate need that this PR addresses is avoiding the introduction of more unicode in the integrators that would soon be deprecated. That was the main blocker to Sunny 0.5.9.