Add warm restart to AJDC

[qbarthelemy]

This PR enhances AJDC when used for group BBS (gBSS): it allows to refine group sources for a specific subject, using an initial diagonalizer.

• Add parameter 'init' to ajd_pham and rjd to allow warm restart, and complete tests;
• Add a warning if 'dim_red' is let to None in AJDC, and add key 'warm_restart' for dimension reduction, and update tests.