ENH: define singleton-style lookups for runTime selection, alias handling

- this makes the lookup and use of tables slightly cleaner and
  provides a hook for update (compat) messages

  The singleton-style method returns the function pointer directly,
  or nullptr on not-found.

  NEW access method (mnemonic: 'ctor' prefix for constructors)

  ```
  auto* ctorPtr = dictionaryConstructorTable(modelType);

  if (!ctorPtr)
  {
      ...
  }

  return autoPtr<myModel>(ctorPtr(dict, ...));
  ```

  OLD method, which also still works, but without any compat handling:

  ```
  auto ctorIter = dictionaryConstructorTablePtr_->cfind(modelType);

  if (!ctorIter.found())
  {
      ...
  }

  return autoPtr<myModel>(ctorIter()(dict, ...));
  ```