ENH: add get() accessor to tmp classes

- similar to autoPtr and unique_ptr. Returns the pointer value without
  any checks. This provides a simple way for use to use either
  an autoPtr or a tmp for local memory management without accidentally
  stealing the pointer.

  Eg,

     volVectorField* ptr;
     tmp<volVectorField> tempField;

     if (someField.valid())
     {
         ptr = someField.get();
     }
     else
     {
         tempField.reset(new volVectorField(....));
         ptr = tmpField.get();
     }

     const volVectorField& withField = *ptr;

STYLE: make more tmp methods noexcept