Raise error when trying to write an old-style group adjlist because its not implemented.
Raise error when trying to write an old-style group adjlist because its not implemented.

For future work we should implement it but it's not a high priority item.

comocheng/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,enochd/RMG-Py,enochd/RMG-Py,nyee/RMG-Py,nickvandewiele/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,pierrelb/RMG-Py