Begin to introduce vectorisation in the model

Further to my comment in https://gitlab.cern.ch/cara/cara/-/issues/103#note_4047752, and as was handled in Markus' monte carlo work, this MR makes the model support arrays as well as scalars.

This MR does not complete the work, but implements the necessary scaffolding and all subsequent changes should be predictable and easy to review (and atomic). Whilst the model isn't yet fully complete they can be merged immediately since they are entirely backwards compatible - the model results do not change as a result of this MR.