fieldAverage: Added periodicRestart option and rationalized naming of restart options

    When restarting form a previous calculation, the averaging is continuous or
    may be restarted using the \c restartOnRestart option.

    The averaging process may be restarted after each calculation output time
    using the \c restartOnOutput option or restarted periodically using the \c
    periodicRestart option and setting \c restartPeriod to the required
    averaging period.

    Example of function object specification:
    \verbatim
    fieldAverage1
    {
        type fieldAverage;
        functionObjectLibs ("libfieldFunctionObjects.so");
        ...
        restartOnRestart  false;
        restartOnOutput   false;
        periodicRestart false;
        restartPeriod   0.002;
        fields
        (
            U
            {
                mean            on;
                prime2Mean      on;
                base            time;
                window          10.0;
                windowName      w1;
            }
            p
            {
                mean            on;
                prime2Mean      on;
                base            time;
            }
        );
    }
    \endverbatim

    \heading Function object usage
    \table
        Property        | Description           | Required    | Default value
        type            | type name: fieldAverage | yes |
        restartOnRestart  | Restart the averaging on restart | no | no
        restartOnOutput   | Restart the averaging on output | no | no
        periodicRestart | Periodically restart the averaging | no | no
        restartPeriod   | Periodic restart period | conditional |
        fields          | list of fields and averaging options | yes |
    \endtable