Finalise Parson's input

[smmercuri]

In this PR we polish the Parson's input. Namely Maxima CAS calls are embedded inside the input type to streamline functionality:

• The model answer is now input as [ta, proof_steps] compared to parsons_answer(ta, proof_steps).
• If "hideanswer" is not used as an Extra option, then a proof display using proof block CSS styling is displayed automatically. Previously this required manual use of the proof_display function by the author.
• Update the documentation on authoring Parson's questions.

[sangwinc]

Thanks @smmercuri, much appreciated. This is really coming together now.