Feature/issue 117 question sequence builder

What's done:
* generic `SequenceBuilder` that can be used for probes and questionnaires. Fixes #117.
* corresponding grammar definition (still evolving a lot)
* corresponding layout inflation

New problems: #160, #161, #162, #163, #164.

Greatly facilitates: #114, #118, #119, #120.

Still to do:
* Implement the interesting features of grammar v3:
   * the new `MatrixChoice` question (spanning two pages)
   * conditionals (#122)
* Finish defining the grammar v3 (look over at [parameters/dev/grammar-v3](https://github.com/daydreaming-experiment/parameters/blob/master/dev/grammar-v3/production.json) for an example of what is done)

I'm merging because it mostly works (modulo new problems), and it will start diverging too much from develop otherwise.

@vincentadam87 , leaving you to merge when you want.