Abort CGSolver when the defect is invalid

The first commit introduces the exception `SolverAbort`, derived from `ISTLError`.

The second commit makes `CGSolver` abort by throwing `SolverAbort` when the defect becomes invalid (infinite or NaN).  The check is done in every iteration before the convergence check.

These two commits fix #12.

The third and fourth commit make `BiCGStab` and `GMRes` throw `SolverAbort` when they detect a breakdown.  `SolverAbort` seemed more specific when the `ISTLError` they were throwing before.

The fifth commit documents when `SolverAbort` is thrown at the moment.

The sixth commit adds a unit tests that tries to trigger `SolverAbort` and makes sure that it is actually thrown.  It does this for the NaN-check in `CGSolver` and for the "abs(h) < EPSILON"-check in `BiCGSTABSolver` -- there a more conditions in `BiCGSTABSolver` and `RestartedGMResSolver` that throw, but I don't know how to trigger them.



See merge request !36