Add documentation stating requirements for valid input data

[sadielbartholomew]

Provide a document which covers, clearly, what form input data, both for the model and observational cases, is required for the end-to-end script to work.

This is important because users need to be able to understand what input data form they need to use the toolkit, so they can pre-process it accordingly (this is not our job), but equally we need to be able to know this clearly so that we can determine whether an issue arising in the toolkit is due to a bug or deficiency in the script, or whether it is due to the input data not conforming to requirements.