2 - JD base Solver

[NicoRenaud]

The JD/Davidson implementation is focused in the class

Implement a JD_SymSolverBase base class

• it also has the functions eigenvalues() and eigenvectors().
• The base class also has setters for settings : max_searchspace,num_iterations, convergence_criteria, ...
• JD_SymSolverBase implements a compute(neigen, target) method, which returns the number of converged eigenvalues and stores the results in eigenvalues_ and eigenvectors_ member variables.
• It also sets the flag info_ which can be accessed by the info() getter, which informs about the success of the calculation. It returns a Specta::CompuationalInfo enum.
• The input parameters for the compute method are, number_of_eigenvalues and the Spectra::SortRule
• it has a pure virtual function, to compute the correction vector called get_correction_vector