Derivations should be able to contain Figures

[bmaclach]

Currently a Derivation in Drasil is a synonym for a [Sentence], so it can only contain things that can be expressed as Sentence. It is easy to imagine wanting a picture to help explain a derivation, but currently such a Figure could not appear in the Derivation and would instead need to appear elsewhere and then be referenced in the derivation, which is not ideal for readability.

[smiths]

Sounds good @bmaclach. As we discussed yesterday, we don't need Figures in our derivations yet, but at some point we will. For instance, the derivation of the moment equilibrium in SSP that we have discussed would eventually benefit from a figure to show the locations of the forces, water table, coordinates, etc.

[JacquesCarette]

Derivation needs an actual design. Right now, as [Sentence], it is basically opaque and unstructured. We don't want to straight-jacket things either, but still, a little more structure would be good!

[balacij]

This ticket is a 'problem' and #968 is a 'jump to the solution' -- we can/should repurpose that one. Closed in favour of discussion continuation in #968.