ENH: runTimePostProcessing - added option to clear/remove objects after use

When specifying line and surface function-object-based visualisation, use the optional clearObjects flag to indicate that source objects should be removed/cleared after use.

Test case: cavity.tgz

Syntax:

    surfaces
    {
        cuttingPlane1
        {
            type            functionObject;
            functionObject  cuttingPlane;
            clearObjects    yes;              // new option
            ...

Note: only files that have been used will be removed, e.g. if a function object has created multiple surface files, unused files will remain at the end of the run - in the attached case the p surface remains...