Let PreconditionedLocalSolver store the local matrix

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.