OpenMP support

[sivetic]

Visual C++ compiler only supports OpenMP 2.0, which stipulates that the for directive must use an int variable. ContractionCleanup.h (line 174) uses an unsigned, which fails to compile on Visual C++. To stay compatible with Visual C++, the loop variable will need to be changed to an int. Code:

#pragma omp parallel for
        for ( int i = 0; i < _graph.size(); i++ ) {
...