Adding more to BlElements basics with an exercise

[StevenCostiou]

I added more basics to elements:

• composition
• position
• individual space instances
• why space roots can raise exceptions if we add multiple times the same element as child of different roots

I do not know if the exercise is well placed, but it can be moved.
It has interest of being close to the BlElement definition.

[Ducasse]

Tx steven this is good to add material we can always modify it later