Paying back technical debt

[fjebaker]

Using this issue to track commits and PRs that have accrued substantial technical debt, and forward planning on how to pay that debt back

• Extended coronal models #204 adds new radial emissivity profile type that doesn't fit into the current emissivity_at interface. Also adds a new 2D transfer function integration routine that doesn't match quite with the existing. Also has no test cases.
• 07e7a1a found that DataInterpolations.jl was no longer type stable enough for us to use. Will need to tag a compat entry for DataInterpolations.jl when the next tagged version is released.