fix potential segmentation fault

If one table and one tree were requested as output from
vtkRCalculatorFilter, its output would be a vtkTable or vtkTree.
It actually needs to be a vtkMultiPieceDataSet to contain both
objects though, and this would result in a segmentation fault
shortly thereafter.  This commit fixes this degenerate case
and adds a test for it.

Change-Id: I53e468bba2e2d336dc7a4a7f9fb53fdcb64f98b6
change the way we shut down the R interface

The use of atexit() was causing R code that uses the parallel
library to fail nondeterministically.

Change-Id: Iff8eaafd786341f7bc10d50338c50525be492385
Merge topic 'vtkr_segfault' into master

21d92cf5 change the way we shut down the R interface
b77eff3c fix potential segmentation fault