Rationalized the keyword to specify a file name in a dictionary to 'file'

e.g. in tutorials/heatTransfer/buoyantSimpleFoam/externalCoupledCavity/0/T

    hot
    {
        type            externalCoupledTemperature;
        commsDir        "${FOAM_CASE}/comms";
        file            "data";
        initByExternal  yes;
        log             true;
        value           uniform 307.75; // 34.6 degC
    }

Previously both 'file' and 'fileName' were used inconsistently in different
classes and given that there is no confusion or ambiguity introduced by using
the simpler 'file' rather than 'fileName' this change simplifies the use and
maintenance of OpenFOAM.