functionObjects::writeObjects: Added option "writeOption"

Description
    Allows specification of different writing frequency of objects registered
    to the database.

    It has similar functionality as the main time database through the
    \c writeControl setting:
      - timeStep
      - writeTime
      - adjustableRunTime
      - runTime
      - clockTime
      - cpuTime

    It also has the ability to write the selected objects that were defined
    with the respective write mode for the requested \c writeOption, namely:
      - \c autoWrite - objects set to write at output time
      - \c noWrite   - objects set to not write by default
      - \c anyWrite  - any option of the previous two

    Example of function object specification:
    \verbatim
    writeObjects1
    {
        type        writeObjects;
        libs        ("libutilityFunctionObjects.so");
        ...
        objects     (obj1 obj2);
        writeOption anyWrite;
    }
    \endverbatim

Patch contributed by Bruno Santos
Resolves bug-report http://bugs.openfoam.org/view.php?id=2090