Regenerating documentation

Please create a virtual environment and use the requirements.txt file to install the necessary bits.

In most cases, running make html should be sufficient to regenerate the documentation. However, if any changes to nml.py have been made, the nml-core-docs.py file in the helpers directory will also need to be run. This script manually adds each class from nml.py to the documentation as a sub-section using the autoclass sphinx directive instead of the automodule directive which does not allow us to do this.