Changed. For the empty prefix, it depends how we use this class. For example, if there is only one PDE to be solved in a computation, we can do like this in the future coding:
```
if(size_of_PDEs==1)
{
   prefix = "";
}
PETScLinear Solver  ls(A, prefix);
```
Anyway, I added a default value for the prefix argument of the constructor.