Remove omega0 from explicit inclusion in optical_element

[rob-shalloo]

I've made a suggested modification to the optical_element class to remove omega0 from explicit inclusion in the method amplitude_multiplier.

I find it somewhat confusing that we have both omega and omega0 as input arguments. Seeing them both, I would automatically assume that the frequency axis omega is then actually centered around 0, and that omega is rather $\Delta \omega$ such that for each optical element we would need to use omega + omega0 but this is not the case. Instead all but one of our optical elements do not use omega0 at all.

I understand that this was specifically introduced in #261 for the case of polynomial spectral phase #263 . Here I believe we should instead add omega0 as an argument required for initialisation of this optic. This optic adds spectral phase as a polynomial expansion of frequency and so defining the central frequency explicitly is, I think, a good idea.

In any case, as mentioned this is just a suggestion so totally fine to not make the change if people prefer it the way it is. Everything mechanically works great as is

@soerenjalas , could you take a look and see what you think, given you created the two PRs above

[MaxThevenet]

Thanks! That looks good to me.
@soerenjalas any comment?