ENH: momentum field function (issue #1105)

    Calculates linear/angular momentum, reporting integral values
    and optionally writing the fields.

Example
    momentum1
    {
        type        momentum;
        libs        ("libfieldFunctionObjects.so");
        ...
        writeMomentum   yes;
        writeVelocity   no;

        cylindrical     true;
        origin  (0 0 0);
        e1      (1 0 0);
        e3      (0 0 1);
    }