liquidProperties: simplified and generalized the IO

When liquids are constructed from dictionary the coefficients are now first
initialized to their standard values and overridden by the now optional entries
provided in the dictionary.  For example to specify water with all the standard
temperature varying properties but override only the density with a constant
value of 1000 specify in thermophysicalProperties

liquids
{
    H2O
    {
        defaultCoeffs   no;

        H2OCoeffs
        {
            rho
            {
                a 1000;
                b 0;
                c 0;
                d 0;
            }
        }
    }
}