Don't bother printing default multiplicity values in toAdjacencyList
fromAdjacencyList returns multiplicity even when reading groups.

This may be None, in which case it is unchanged (default is [1,2,3,4,5])
Added fromOldAdjacencyList method.

It is mostly copied and pasted from before this branch,
but updated to try and create molecules and groups of the new type.
It tries it if the fromAdjacencyList fails.

The hardest part (perhaps not perfect) is how to decide on a molecule
(or group) multiplicity, given that the old style adjacency list stored
these as atomic properties (eg. 2S, 2T, etc).
Group.toAdjacencyList needs to pass multiplicity.