ENH: IOobjectList simpler construction of unregistered

- accept IOobjectOption::registerOption with (MUST_READ, NO_WRITE)
  being implicit. Direct handling of IOobjectOption itself, for
  consistency with IOobject.

  The disabling of object registration is currently the only case
  where IOobjectList doesn't use default construction parameters,
  but it was previously a bit awkward to specify.

applications/utilities/parallelProcessing/decomposePar/decomposePar.C

@@ -887,9 +887,7 @@ int main(int argc, char *argv[])
                         mesh,
                         runTime.timeName(),
                         cloud::prefix/cloudDir,
-                        IOobject::MUST_READ,
-                        IOobject::NO_WRITE,
-                        false
+                        IOobject::NO_REGISTER
                     );
 
                     // Note: look up "positions" for backwards compatibility
@@ -972,9 +970,7 @@ int main(int argc, char *argv[])
                             mesh,
                             runTime.timeName(),
                             cloud::prefix/cloudDirs[cloudI],
-                            IOobject::MUST_READ,
-                            IOobject::NO_WRITE,
-                            false
+                            IOobject::NO_REGISTER
                         );
 
                         lagrangianFieldCache.readAllFields