This commit introduces a new small molecule topology graph for 
adding `N` arms to a central building block with `N` functional 
groups. It is complicated a bit by allowing the same flexibility and 
ordering provided by the linear polymer topology graph.

Currently, I am proposing that the core is aligned into one plane. Then
the arms are aligned, by their user-prescribed pattern, defined by the
rotation around the core.

I have also added minor things to the source code as I went along, fixed
a typo, started adding clearer explanations of exceptions. These can be
moved to another PR, but they came out naturally in this process.