[Technical assessment] AcousticMaterialBase objects instantiated for each triangle

User story :
As Tympan Team, I want to analyze the design choice of duplicating AcousticMaterialBase objects for each triangle, in order to improve design and performance.

This issue relates to this issue note.

Acceptance criteria :

• document current design with strength and weakness,
• if relevant, study an alternative
  • estimate gain, performance, memory consumption, reliability, ...
  • estimate necessary effort to implement.