ENH: add dictionary::findStream() - symmetric with findDict()

- can be used with this type of code:

  ITstream* streamPtr = dict.findStream(name);
  if (streamPtr)
  {
      auto& is = *streamPtr;
      ...
  }

  versus:

  const entry* eptr = dict.findEntry(name);
  if (eptr && eptr->isStream())
  {
      auto& is = eptr->stream();
      ...
  }

ENH: add findStream(), streamPtr(), isStream() to dictionary search

- symmetric with findDict(), dictPtr(), isDict() methods

STYLE: use findDict() instead of found() + subDict() pairing

COMP: define is_globalIOobject trait at top of IOobject header

- more visibility, permits reuse for specializations etc.