step-19: Revise discussion about time stepping + update movie

[kronbichler]

As discussed in #10942 (comment) , the tutorial program is a bit inconsistent in the convention for the time step. In particular, I believe that the result section should not call the method "forward Euler" because the formula is indeed using the end-of-step electrical potential/field, but it might be that something else was intended anyway. Given #10942, at least the formulas in the introduction and in-code comments should be consistent with what is implemented.

[kronbichler]

Furthermore, I get a different motion of particles with #10942, so I believe the movie needs to be updated as well.

[peterrum]

This does not seem urgent.

[bangerth]

I'd like to get this done, given that step-19 is new in this release.

[peterrum]

I'd like to get this done, given that step-19 is new in this release.

@bangerth It would be nice if the documentation would be updated. The movie is not that important.

[kronbichler]

I guess this is a very minor topic and does not distract from the main innovation of the tutorial, so I simply adjust the milestone for now. If we have a last-minute fix, I will be fine with it as well.

[kronbichler]

@bangerth do you see a chance to get this done for the release? I do not think this is super urgent, so we might also remove the target milestone and/or close the issue.