Matrix-less build reduced problem in BCG

[matbesancon]

We materialize a matrix M in build_reduced_problem only for an inner product x^T M y in the next function.

This matrix cannot be used in-place because its size depends on the number of atoms in the current active set.

Instead of building it, we should create a custom matrix type that:

• lazily wraps the active set
• implements dot(x, M, y) by looping through the active set