ENH: improve objectRegistry functionality (issue #322)

• Recursive searching for objects within a registry is now optional (previous it was always done).

  A recursive search effectively blocks the construction of sub-sub-registries if their names are 'masked' by some parent level sub-registry with the same name! (BUG)

• Recursive search is now turned OFF by default, which makes it consistent with dictionary and probably causes the least number of surprises.

Various new convenience methods added:

lookupObjectRef()

• returns a non-const reference. For example,

  volScalarField& U = mesh().lookupObjectRef<volScalarField>("U");

  Instead of

  volScalarField& U = const_cast<volScalarField&>
  (
      mesh().lookupObject<volScalarField>("U")
  );

-- lookupObjectPtr()

• returns a const pointer, and nullptr on failure. For example,

  const volScalarField* Uptr = mesh().lookupObjectPtr<volScalarField>("U");
  if (Uptr)
  {
      const volScalarField& U = *Uptr;
      ...
  }

  Instead of

  if (mesh().foundObject<volScalarField>("U"))
  {
      const volScalarField& U = mesh().lookupObject<volScalarField>("U");
      ...
  }

-- lookupObjectRefPtr()

• returns a non-const pointer, and nullptr on failure. For example,

  volScalarField* Uptr = mesh().lookupObjectRefPtr<volScalarField>("U");
  if (Uptr)
  {
      volScalarField& U = *Uptr;  // use as reference
      (*Uptr) = ...;              // or use directly
  }

  Instead of

  if (mesh().foundObject<volScalarField>("U"))
  {
      volScalarField& U = const_cast<volScalarField&>
      (
          mesh().lookupObject<volScalarField>("U")
      );
  }

-- sortedNames()

• now works with template parameters and with regular expression matching as well. For example,

  wordList names  = mesh().sortedNames();
  wordList fields = mesh().sortedName<volScalarField>();

  Instead of

  wordList names  = mesh().sortedNames();
  wordList fields = mesh().names<volScalarField>();
  Foam::sort(fields);

--