Write ExemplarCreation workflow

[arnaudon]

It will take as input morphologies, and create an exemplar per mtype. Various variants should be possible, and some validation plots will be needed.

Current variants to be implemented:

• placeholder model with single 'equivalent' compartment
• detailed exemplar with average local surface area profile

[arnaudon]

https://github.com/BlueBrain/morphology-workflows/pull/66