Use model builder to place the test alkanes - the AtomPlacer3D is only for placing chains.

Change-Id: I554dda109a5ed5fbd6a4b015e610eb6027b08437
Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Throw an exception when a non-chain is provided. Documentation was also updated.

Change-Id: I59d802abb924d328516f06f9216759bdea30dd25
Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Test that providing a non-chain as a chain throws an exception.

Change-Id: I4eaf8883bc7f8da7a743091b65006fdb4eea852e
Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>