[Feature Req] k/mu conversion from kperp/kpara

[steven-murray]

Description

Theorists sometimes use |k|, \mu instead of k_perp, k_par in computing the theory. It might be nice to have convenience methods to support this.

The Solution I'd Like

I think the best solution might be to provide a @property for mu that can be used if desired. Then have a flag that tells the DataModelInterface whether the theory function uses k/mu or kperp/kpar. Something like theory_uses_mu=True.

Alternatives Considered

The conversion between the two is obviously very easy, and we could just leave it up to the theorist to write their function in such a way as to do the conversion. Whether providing a convenience method is a good idea would be dependent on how likely it is for a theorist to want to use the k/mu basis.

Additional context

The downside of providing the convenience is essentially that there is another parameter, and more documentation. We should balance this against the utility of it.