ENH: residuals function object - extended to write residual fields

Residual fields can be written using the new 'writeFields' entry, e.g.

    functions
    {
        residual
        {
            type            residuals;
            libs            ("libutilityFunctionObjects.so");
            fields          (".*");
            writeControl    writeTime;
            writeFields     true;
        }
    }

Fields currently correspond to the initial residual for the last solver
iteration.