The preconditioned local solver needs the local block of the
linear elasticity matrix, for use as its quadratic model.
Previously, this matrix was extracted from the global
Hesse matrix, with some tricks to account for the scaling
of that global matrix with the residual stiffness k.

With this patch, there is now a dedicated local solver object
for each vertex, and each objects stores the correct local
matrix.  This simplifies the code a little.