elm-pb example: Adding relaxing phi boundaries

Adds an option phi_boundary_relax. If set to true, then the radial
boundary conditions on the potential phi are relaxed over a given
timescale towards zero gradient.

Adapted from Hermes-3
(https://github.com/boutproject/hermes-3/blob/master/src/vorticity.cxx#L261).

Author: bendudson