#1535 In PETSc wrappers, make Internal() method a non const method in Vector and Matrix class
Currently it is const (and on the paper it is: no attribute is formerly modified in this method...) but it might lead to functionaly coounter-intuitive situation (for instance MatShift function which works on a constant GlobalMatrix...
It would be better to make it non constant, even if this means providing another one for const methods (but the risk would be nonetheless mitigated as there would be an evaluation to see if really the const status is functionally achieved or not...