Make runnable in userspace

The MongoDB dependency requires that the system be running a Mongo service, which is an installation barrier. Maybe it's not an issue and we'll expect users to e.g. run locally via a Docker image, but I can see this also being trouble for us if we want to run a "global" version via a PPE hosted web server. Worth thinking about, along with whether the HDF5 HepMC format could replace a db entirely (since HDF5 is also "table based" cf. PyTables)