Make compatible with NEURON9.0

[darshanmandge]

• NEURON 9.0 was released last week:
  https://github.com/neuronsimulator/nrn/releases/tag/9.0.0
  https://pypi.org/project/neuron/9.0.0/#files
  https://nrn.readthedocs.io/en/latest/changelog.html
• There are some breaking changes.

[darshanmandge]

Solved by #67.