liquidProperties: Simplified dictionary format

The defaultCoeffs entry is now redundant and supported only for backward
compatibility.  To specify a liquid with default coefficients simply leave the
coefficients dictionary empty:

    liquids
    {
        H2O {}
    }

Any or all of the coefficients may be overridden by specifying the properties in
the coefficients dictionary, e.g.

    liquids
    {
        H2O
        {
            rho
            {
                a 1000;
                b 0;
                c 0;
                d 0;
            }
        }
    }