Add ability to load external particle fields from file

[oshapoval]

Description of PR #4981 @RemiLehe:

We currently have the ability to load an external grid field from a file (thanks to #3584).

This PR provides a similar ability for the external particle fields.

The external grid and particle fields differ e.g. in the case of the EM solver:

• The external grid fields are applied to the grid at t=0 and serve as initial condition to the Maxwell solver.
• By contrast, the external particle fields are applied to the particles directly and are not affected by the Maxwell solver.

In this PR, support for external particle fields is achieved by reusing many of the structures that are used for the external grid fields. For instance, we store the fields from file into the field Efield_fp_external and Bfield_fp_external in both cases. (As a result, using external particle fields and external grid fields from file at the same time is not possible.) However:

• in the case of the external grid fields, Efield_external is added to the grid fields Efield_fp directly.
• in the case of the external particle fields, Efield_external is instead added to the fields from which the particle gather Efield_aux. In some case (e.g. energy-conserving gather), Efield_fp and Efield_aux used to be aliases of each other. This is no longer the case if the external particle fields from file are used.

[roelof-groenewald]

It would be good to include the divergence cleaning from #4944 to these particle fields as well.