WritePrimaries option writes first sampler events in the Primaries branch.

[William Shields]

When specifying the option writePrimaries=0, the events for the first sampler are stored in the primary branch, with the sampler branch being empty.

An example file for recreating this is attached. The primaries themselves (reference particle with no offset but with finite xp0) are not recorded, but the primary branch will contain the particle with the finite x position that you would expect to see in recorded in the sampler.

[William Shields]

Fixed and tested in the develop branch.