adjusted dictionary #inputMode directive

- #inputMode error
  now issues a FatalError on duplicate entries

- #inputMode warn
  issues a warning on duplicate entries, corresponds to the
  old behaviour of 'error'

- #inputMode protect
  prevents overwriting existing entries

The 'protect' mode provides a simple mechanism for supplying default values.
eg,
    in file1:
        #inputMode  protect
        intensity       0.1;
        mixingLength    0.005;
        #inputMode  merge

        inlet
        {
            type        turbulentIntensityKineticEnergyInlet;
            intensity   $intensity;
        }

    which is included from file2:

        intensity   0.05;
        #include    "file1"