Requirements for HIPI

This will formalise the goals and constraints for the HIPI, specifying both functions that it should perform and constraints its design should obey to be acceptable to its intended users. These requirements will be prioritised, identifying those that are critical (and must be satisfied), and those that are desirable (and should be satisfied, but not necessarily). This will serve both as a guide for technical development in WP5, and as the basis for evaluating the final device in WP6.