- make centrally available via #include "addOverwriteOption.H"

- to match coding guidelines

- ensure that the standard options (eg, from timeSelector) also have
  some usage information

- also drop various unused time options from src/OpenFOAM/include

- removed the previously added fileName type(), isDir(), isFile() methods.

- added optional bool parameter to isFile() function to explicitly
  enable/disable the check for gzip files.

- fixed minor bugginess where the default usage of isFile() would result in
  false positive matches.

- be slightly more stringent and use isDir() and isFile() instead of
  exists() function when we actually know the expected type.

- boundBox::invertedBox is useful for initializing our own calculations
- NOTE treeBoundBox::greatBox is still in place, since it uses GREAT
  instead of VGREAT. If this is only historical, we can drop it.