Factorize EigenMatrix

[Franco Milicchio]

It would be considerably useful to let users factorize an EigenMatrix in order to re-use it with different RHSs, for instance using the EigenLUSolver class. The current implementation, however, does not allow calling the set_operator method on non-const matrices:

/// Set operator (matrix)
void set_operator(std::shared_ptr<const GenericLinearOperator> A);

/// Set operator (matrix)
void set_operator(std::shared_ptr<const EigenMatrix> A);

[Jan Blechta]

I believe that constness is not an obstacle here. The factorization would be stored within the Eigen solver class, not the matrix.

[Jack Hale]

Merged in mliertzer/dolfin/fix-eigen-lu-refactorisation (pull request #477)

Fix issue 847 - Cache factorization in EigenLUSolver

→ <<cset acc9f13e09ef>>

[Chaffra Affouda]

I think this change leads to some memory leak or overrun. Objects are not being deleted properly. Please see issue 1028