Also extended the cubic equation test routine and modified the error
methods so that they more accurately generate the round of error of
evaluation.

This resolves bug report https://bugs.openfoam.org/view.php?id=3015

- missing indexing caused the real radius to be largely ignored
  when useRealRadius = true

- the original intention was to avoid vtp output when the clouds are
  empty anyhow. However, it is useful for post-processing to have
  clouds with zero parcels (eg, before the start of injection).

  Pruning of empty clouds is now an option in the vtkCloud dictionary
  controls, with the default being false (no pruning). This represents
  a non-breaking change in behaviour since it generates more output
  than previously.

  Example,
  {
      type    vtkCloud;
      //- Suppress writing of empty clouds (default: false)
      prune   true;
  }

- use "min:xxx" format for AMIInterpolation as well

Merge branch 'develop-pre-release' of develop.openfoam.com:Development/OpenFOAM-plus into develop-pre-release

- the backend script should not be accessible from the PATH.
  Using deprecated/ adds a level of documentation.

- functionObjectLibs -> libs
- redirectType -> name
- change deprecated writeCompression flags types to Switch.

- cleanup some trailing ';;' from some dictionaries

STYLE: separate areaNormal/unitNormal method for primitives (issue #885)

See merge request OpenFOAM-plus!208

- regularise referencing of OpenFOAM env variables, sources etc.

- The problem is not 100% reproducible, but be somewhat conservative
  and now only close(stdin) for pure background (daemon-like) use of
  Foam::system(), where it can be useful.

  Using Foam::system() for launching background processes is currently
  only used for the doc browser.