More pedagogical notebeooks

[glyg]

Remark from https://github.com/openjournals/joss-reviews/issues/2973#issuecomment-771524127

My main concern is the lack of gradual difficulty in the tutorials : reproducing research results is great, but gently steering the user might be missing.

While example 5 is simple as claimed, example 2 is rather complex. To engage scientists to use the software, step-by-step tutorial, or a better gradation of difficulty in the notebook examples, seem required to smoothen the steep learning curve.

[glyg]

New organisation:

• [x] Bases

  Bases

  Creation

  Data structure

  Specification

  I/O

• [x] Geometry

  Geometry

  2D

  2.5D

  Closed 2.5D

  Monolayer

  Closed Monolayer

  Bulk

• [x] Visuallization

  Visuallization

  Draw specs (document callable "color" spec)

  2D

  3D

• [x] Effectors & Models

  Effectors & Models

  TODO (from the Energy min & qsolver nbs)

• [ ] Solvers

  Solvers

  History

  Quasistatic

  Euler

• [ ] Division and rearangements

  Division and rearangements

  Cell division in2 & 3D

  Type 1 & I-O-H

  Rosettes & atomic rearangements

• [ ] EventManager TODO (see #6)