[Ref config] Improve mesh adaptation robustness.

At each mesh adaptation step, coordinates have to be updated to
    X = coordinates + displacement.
Then the SCOREC library uses these coordinates (plus other data) to adapt the
mesh. The problem is that X may have negative element volumes.

As a solution, implement a loop in AAdapt::MeshAdapt that, when necessary,
incrementally achieves
    X = coordinates + displacement
by a sequence of steps
    Set X = coordinates
    Loop on i:
      Set X += alpha_i displacement.
      Adapt mesh.
where sum_i alpha_i = 1. At each iteration of the loop, X has only positive
element volumes.