Binder notebooks fail on import of ctd

[callumrollo]

Attempting to run notebooks from binder link in README gives

ModuleNotFoundError: No module named 'ctd'

Likely due to lack of ctd in .binder/environment.yml

Alternatively, one could add the parent directory to the path in the notebooks to use the local version of ctd

[callumrollo]

Resolved by #179