Test if an atom is present in a stereo element.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
When removing attempting to remove an atom and all connected parts - we now also update the IStereoElements. This linear search is inefficient but matches the style of what was already done and is available in the AtomContainer.

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