fileName gets additional convenience methods

- 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.