[WIP] Extending preconditioners to handle a diagonal operator for E-field

[debog]

• Extended the CurlCurlMLMGPC interface to work with non-scalar beta coefficient. It can now be a MultiFab array (same size/layout as E-Fields) with one component representing a diagonal matrix/operator on the E-field.

• Implemented a JacobiPC preconditioner to solve the system \sigma E = b using the Jacobi iterations. For now, \sigma is a diagonal matrix (stored as a MultiFab array (same size/layout as E-Fields) with one component. The solution is set as E = b / \sigma, so the iterations are not yet implemented. The complete Jacobi method will be implemented once non-diagonal elements of \sigma are available.

The preconditioners fetch a pointer to the matrix operator (beta or \sigma) from the implicit time integrator object.