Add possibility to pass the startVector to compute eigenvectors

[jhabriel]

Currently, the solvers adaptive_lgmres-e,gmeres-e and lgmres-e use the built-in function eigs to compute eigenvalues. eigs, by default, initialises the start vector with random values. It is possible, however, to pass the StartValue to eigs. Thus, we should extend the signature of the solvers to allow for this option. This parameter should be optional.

The motivation behind this is to have full control on the final number of iterations, which is particularly relevant for testing.