Hi Remi,

I had to modify tests/test_linear_wakefield.py to increase p_zmax. Plasma was previously defined for 39e-6 < z < 41e-6, but since zmax = 40e-6 the plasma extent is only used for the initial loading in the full domain. The continuous injector doesn't check, and so continued loading never-ending plasma as the window moved. The above PR does now check the plasma extent, and so only loaded 2 microns of plasma then vacuum again, so the assert comparisons for the wakefield were bound to fail.

I have reverted to this implementation (which exploits short-circuit evaluation):

Npz = 0 if (self.p_extent is not None and len(self.p_extent)>1 \
            and (zmax < self.p_extent[0] or zmin > self.p_extent[1])) \
        else self.nz_inject

but I would be happy to split it into nested if statements if preferred.

Sorry for the very late review!
After looking at the code more closely, I think that this would require some small modifications in order to work properly in the boosted frame and/or with a moving plasma.
I will modify the PR accordingly in the next few days.