Idempotence of adaptation algorithm

[Algiane]

Idempotence of Mmg algo (that is, Mmg doesn't change the mesh if it is already a computational mesh and if it is suitable in term of edge lengths and qualities) should improve:

• load balancing after first iteration (mesh is almost entirely adapted at first iter)
• efficiency: Mmg will have almost no work after first iteration

Idempotence may be influenced by:

• type or metric (isotropic / anisotropic)
• gradation
• required gradation