Field of MovingCharges continual update

Following Todd Zimmerman's tutorial[ _Getting Started Animation with Manim and Python 3.7_](https://talkingphysics.wordpress.com/2019/01/08/getting-started-animating-with-manim-and-python-3-7/) on Chapter 10, he presents continual update for vector fields.
Everything previously worked perfectly. However continual update doesn't seem to work.
Other similar issues on the Github Issues has returned an error, however it is not my case.
In my case, the code runs fine, but the particles aren't moving and are in the starting position. https://youtu.be/ZbDjQbl5mzg It seems like some other users have had the same issues in this video's comments.

I am on Windows and using Python 3.8.3.

The code is the exact same from the tutorial (with the import in-front):

```
[class MovingCharges(Scene):
CONFIG = {
"plane_kwargs" : {
"color" : RED_B
},
"point_charge_loc" : 0.5*RIGHT-1.5*UP,
}
def construct(self):
plane = NumberPlane(**self.plane_kwargs)
plane.add(plane.get_axis_labels())
self.add(plane)
```

field = VGroup(*[self.calc_field(x*RIGHT+y*UP)
for x in np.arange(-9,9,1)
for y in np.arange(-5,5,1)
])
self.field=field
source_charge = self.Positron().move_to(self.point_charge_loc)
self.play(FadeIn(source_charge))
self.play(ShowCreation(field))
self.moving_charge()

def calc_field(self,point):
x,y = point[:2]
Rx,Ry = self.point_charge_loc[:2]
r = math.sqrt((x-Rx)**2 + (y-Ry)**2)
efield = (point - self.point_charge_loc)/r**3
return Vector(efield).shift(point)

def moving_charge(self):
numb_charges=4
possible_points = [v.get_start() for v in self.field]
points = random.sample(possible_points, numb_charges)
particles = VGroup(*[
self.Positron().move_to(point)
for point in points
])
for particle in particles:
particle.velocity = np.array((0,0,0))

self.play(FadeIn(particles))
self.moving_particles = particles
self.add_foreground_mobjects(self.moving_particles )
self.always_continually_update = True
self.wait(10)

def field_at_point(self,point):
x,y = point[:2]
Rx,Ry = self.point_charge_loc[:2]
r = math.sqrt((x-Rx)**2 + (y-Ry)**2)
efield = (point - self.point_charge_loc)/r**3
return efield

def continual_update(self, *args, **kwargs):
if hasattr(self, "moving_particles"):
dt = self.frame_duration
for p in self.moving_particles:
accel = self.field_at_point(p.get_center())
p.velocity = p.velocity + accel*dt
p.shift(p.velocity*dt)

class Positron(Circle):
CONFIG = {
"radius" : 0.2,
"stroke_width" : 3,
"color" : RED,
"fill_color" : RED,
"fill_opacity" : 0.5,
}
def __init__(self, **kwargs):
Circle.__init__(self, **kwargs)
plus = TexMobject("+")
plus.scale(0.7)
plus.move_to(self)
self.add(plus)
](url)

這個 issue 是關於 continual_update 方法未在向量場中為粒子播放動畫的問題。先檢查 MovingCharges 類別及其 continual_update 方法。查看 manim codebase 中 always_continually_update 旗標的使用方式。尋找 repository 中的類似範例以了解動畫如何被觸發。驗證 frame_duration 和更新迴圈。運行所提供的代碼來重現靇態輸出。

由索引模型根據 Issue 內容生成。