Fix compiler warning in MPICommunicator.

In MPICommunicator.h line 142 - 151:

if (rvalues.size() == 0) {
    rvalues.resize(1);
    rvalues[0] = -999; // FIXME warning C4245: As svalues and rvalues are of type vector<T> there could be a
                       // potenial signed/unsigned mismatch storring a negative number
}
if (scount == 0) {
    svalues.resize(1);
    svalues[0] = -999; // FIXME warning C4245: As svalues and rvalues are of type vector<T> there could be a
                       // potenial signed/unsigned mismatch storring a negative number
}