These atoms need to be recognised for creating extended tetrahedral (allene) stereo elements.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Additional commit to fix still failing test. The previous example was hitting a heuristic in the code, where there is more than one hydrogen the type is automatically set to non-stereogenic. This was the case for the terminal atoms in this extended tetrahedral example.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>