Merge branch 'feature-runtime-post-pro-clean-objects' into 'develop'

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](/uploads/62cc2761d132f42456f2af08f1499eba/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...

See merge request !89