Models automatically evaluate at energy bin edges or centers

[jajmitchell]

Describe the feature

In https://github.com/sunpy/sunkit-spex/pull/202, edges was added to allow the user to evaluate the GaussianModel and StraightLineModel at energy bin edges or energy centers. This is a little clunky.

Proposed solution

This should be automated through the use of the SpectralAxis in the data container.