Add default `select` procedure for `eigs`

[loiseaujc]

At the moment, eigs returns tentative eigenvectors and eigenvalues even if not converged. This is a design problem.

By default, there are not selection procedure and thus no krylov-schur restart. The basis of eigenvectors is pre-allocated. If kdim is not passed, it performs 4*nev iterations before stopping. If kdim is passed, it performs kdim iterations.

Two options:

• either we raise a warning if not select procedure is passed.
• or we use a default selection procedure (e.g. similar to what is done in the associated test).

[loiseaujc]

Closed with #95