- prefer csorted() method for const access since it ensures that the
  return values are also const pointers (for example) even if
  the object itself can be accessed as a non-const.

- the csorted() method already existed for HashTable and
  objectRegistry, but now added to IOobjectList for method name
  consistency (even although the IOobjectList only has a const-access
  version)

ENH: objectRegistry with templated strict lookup

- for lookupClass and csorted/sorted. Allows isType restriction as a
  compile-time specification.