- using 'nil' type to write geometry only, in the future we could fallback to 'null'
  writer and hook in arbitrary formats from surfMesh

- IOobject gets writeEndDivider() static method

- regIOobject: don't re-register an unregister object on rename/assignment
- Hasher: split-off HasherInt with uint32_t specializations
- IOobject: writeBanner/writeDivider return Stream for easier chaining.

... also dropped some namespace bracketing while I was at it.

- exists() = forward to OSspecific exists(...)
- isDir()  = forward to OSspecific dir(...)
- isFile() = forward to OSspecific file(...)
- IOobjectComponents() - split into instance, local, name following rules
  set out for IOobject.

- added IOobject(path, registry, ...) constructor that uses
  fileName::IOobjectComponents(). This hides the complexity we otherwise need.

  - consistent constructors and consistent behaviour with MUST_READ,
    READ_IF_PRESENT

  * new -srcDoc option to display doxygen-processed source code
  * display standard options -help/-doc/-srcDoc at end of usage
  * consolidate code when writing IOobject headers
  * drop writeLogbanner and add optional bool to writeBanner
  * minor formatting changes