ENH: Added new columnAverage function object

    Averages columns of cells for layered meshes.

    For each patch face, calculates the average value of all cells attached in
    the patch face normal direction, and then pushes the average value back
    to all cells in the column.

    Useful for channel-like cases where we want to average fields in the
    spanwise direction.

    Example of function object specification:
    columnAverage1
    {
        type        columnAverage;
        libs        ("libfieldFunctionObjects.so");
        ...
        patches     (front side);
        fields      (U p);
    }

    Where the entries comprise:
    \table
        Property     | Description               | Required    | Default value
        type         | type name: fieldMinMax    | yes         |
        patches      | list of patches to collapse onto | yes  |
        fields       | list of fields to process | yes         |
    \endtable