Improved Communicator class

Now all the MPI wrappers come in two flavors. They can be used
with a pointer as usual, e.g.

double*data;
comm.Sum(&data[0],n);

or they can be used with a reference to a single number

double x;
comm.Sum(x);
// same as comm.Sum(&x,1);

Notably, the second version also works with std::vector and std::string
and automatically detect the size, e.g.

std::vector<double> v;
comm.Sum(v);
// same as comm.Sum(&v[0],v.size());

Furthermore, everything works transparently with Vector and Tensor
either as single objects or elements
of vectors or of C arrays:

std::vector<Vector> v;
comm.Sum(v);
// same as comm.Sum(&v[0],v.size());
// same as comm.Sum(&v[0][0],3*v.size());

All the regtests are satisfied