Bugfix: allow new-style method animation to be used in animation groups

[behackl]

Motivation

New-style method animations (using the .animate syntax) can't currently be included in AnimationGroup or Succession.

Overview / Explanation for Changes

With this change, animations in the list are checked whether they are a _AnimationBuilder -- and if so, the corresponding animation is built and added to self.animations.

Oneline Summary of Changes

- Bugfix: allow new-style method animation to be used in animation groups (:pr:`PR NUMBER HERE`)

Testing Status

Added a test for AnimationGroup and Succession, passing locally.

Acknowledgements

• I have read the Contributing Guidelines

Reviewer Checklist

• Newly added functions/classes are either private or have a docstring
• Newly added functions/classes have tests added and (optional) examples in the docs
• Newly added documentation builds, looks correctly formatted, and adds no additional build warnings
• The oneline summary has been included in the wiki

[friedkeenan]

Perhaps there should be a single function to handle this stuff that could be used in AnimationGroup and in Scene, something like

def handle_animation(anim):
    if isinstance(anim, _AnimationBuilder):
        return anim.build()

    if isinstance(anim, Animation):
        return anim

    raise TypeError(...)

[behackl]

dd54e1f now delegates building of an animation from an animation factory to a dedicated prepare_animation function.

[friedkeenan]

Looks good to me, just have a couple not very important nitpicks