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);

--