Succession should reuse more code of AnimationGroup

Nobody has claimed this yet.

## Enhancement proposal

The current implementation of `Succession` is wildly different from `AnimationGroup`; but I don't think that there are a lot of reasons for that. If `Succession` isn't already covered by creating an `AnimationGroup` with `lag_ratio=1` (maybe because this wouldn't give the correct output if the subanimations have different run lengths?), `AnimationGroup` should be made more flexible so that `Succession` can use the same code.

Old issue description below.

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## Description of bug / unexpected behavior
`Succession` (sometimes?) seems to mess up initial states of mobjects included in the passed animations. See the example below.

## Expected behavior
The initial states should be respected (and in particular: mobjects should not appear before their creation animation is played).

## How to reproduce the issue
In the example below, the blue circle appears before its `FadeIn` animation is played, then disappears and fades in. It should not be visible before its animation.

Code for reproducing the problem

```py
from manim import *
from manim import color as C

class SuccessionTest(Scene):
def construct(self):
c1 = Circle(color=C.RED, fill_opacity=0.7)
c2 = Circle(color=C.BLUE, fill_opacity=0.7)
self.add(c1)
anim = Succession(
ApplyMethod(c1.shift, LEFT),
FadeIn(c2)
)
self.play(anim)
self.wait()
```

## Additional media files
Images/GIFs


Start by comparing the Succession and AnimationGroup implementations and run the SuccessionTest example from the issue. Determine whether AnimationGroup with lag_ratio=1 can preserve differing run lengths and initial mobject states; done means Succession reuses suitable shared logic and the blue circle remains hidden until FadeIn plays.

Written by the indexing model from the issue text.