writeVTK: New functionObject to write fields is VTK format

Description
    This functionObject writes objects registered to the database in VTK format
    using the foamToVTK library.

    Currently only the writing of the cell-values of volFields is supported but
    support for other field types, patch fields, Lagrangian data etc. will be
    added.

    Example of function object specification:
    \verbatim
        writeVTK1
        {
            type        writeVTK;
            functionObjectLibs ("libIOFunctionObjects.so");
            ...
            objectNames (obj1 obj2);
        }
    \endverbatim

    \heading Function object usage
    \table
        Property     | Description             | Required    | Default value
        type         | type name: writeVTK     | yes         |
        objectNames  | objects to write        | yes         |
    \endtable