More options in DispersionFitter and fit constant loss tangent model

[weiliangjin2021]

Addressing #1293 and use FastFitter to fit constant loss tangent material.

I'm not sure about the best way to implement #1293; currently it's using classmethod to load from complex permittivity and loss tangent, while we still only support wavelength but not frequency.

[weiliangjin2021]

One more comment: now that fitter is fast and stable, should we move material fitter from plugin to tidy3d components, so that we can do mat = AbstractMedium.constant_loss_tangent_model(eps_real, loss_tangent, frequency_range) as opposed to mat = FastDispersionFitter.constant_loss_tangent_model(eps_real, loss_tangent, frequency_range)?

[tylerflex]

Implementation looks good to me!

Don't forget mention this in the changelog btw

[tylerflex]

One more comment: now that fitter is fast and stable, should we move material fitter from plugin to tidy3d components, so that we can do mat = AbstractMedium.constant_loss_tangent_model(eps_real, loss_tangent, frequency_range) as opposed to mat = FastDispersionFitter.constant_loss_tangent_model(eps_real, loss_tangent, frequency_range)?

We can consider that, but we still need to support the plugin for backwards compatibility. So I guess is there a big advantage to moving it?

[weiliangjin2021]

One more comment: now that fitter is fast and stable, should we move material fitter from plugin to tidy3d components, so that we can do mat = AbstractMedium.constant_loss_tangent_model(eps_real, loss_tangent, frequency_range) as opposed to mat = FastDispersionFitter.constant_loss_tangent_model(eps_real, loss_tangent, frequency_range)?

We can consider that, but we still need to support the plugin for backwards compatibility. So I guess is there a big advantage to moving it?

We can leave a wrapper in the plugin for compatibility, similar to how we move complex polyslab from plugin to components:

tidy3d/tidy3d/plugins/polyslab/polyslab.py

Lines 3 to 8 in 14dcf5b

	from ...components.geometry.polyslab import ComplexPolySlabBase
	from ...components.medium import MediumType
	from ...components.structure import Structure
	class ComplexPolySlab(ComplexPolySlabBase):

[tylerflex]

I see, if you find this would improve usability, feel free to change it. But at the same time, I'm fine with the way things are now too. Up to you

[dmarek-flex]

Looks good to me, excited to try some new applications.

[weiliangjin2021]

@tylerflex any more comments?

[tylerflex]

no, looks good to me