functionObjects/field/histogram: New functionObject to write the...

functionObjects/field/histogram: New functionObject to write the volume-weighted histogram of a volScalarField

e.g.
    pressureHistogram
    {
        type            histogram;

        functionObjectLibs ("libfieldFunctionObjects.so");

        field           p;
        nBins           100;
        min             -5;
        max             5;
        setFormat       gnuplot;
    }