ENH: Pressure function object - added optional inclusion of hydrostatic pressure

Example - create p-rgh from p:

    pressure-p-rgh
    {
        type            pressure;
        libs            (fieldFunctionObjects);
        writeControl    writeTime;
        mode            static;
        rho             rhoInf;
        rhoInf          1;
        hydrostaticMode subtract;
        g               (0 -9.81 0);
        hRef            0;
    }

- the hydrostaticMode entry is optional - setting is shown during construction
- g and/or hRef values are retrieved from the database if not specified