Then we need to set a place outside of solver class to quit the FEM calculation when a fatal problem (strictly including the case of the  maximum iteration reaches  )  in solver occurs, e.g.
```
bool convergence_status = [solver instance].slove(b, x);
if(!convergence_status)
{
   ...
   exit();
}
```
I do think we like to quit program manually under such situation.

If this is OK, the return value of solve() will be true only if 
its<max_its;