Rework animation index selection

[mwestphal]

Is your feature request related to a problem? Please describe. In order to select all animations at the same time, one has to do currently: --animation-index=-1. This design then trickled down to the animationManager.

This is not the right implementation, this option should be a vector<int>, with empty vector being no animation selected, multiple entries for multiple animations and default should be {0}.

We could consider still supporting any -1 in the vector as a simple select all animations.

Another thing to consider is that some importer support multiple animation, other do not. It may be even better to add a vtkImporter API to provide this information in order to make decisions in the way we present information to the user.

Finally, there is currently a small bug when switching between file with a different number of animations, lets fix that too.

Describe the solution you'd like Rework animation.index implementation as described above

It will then be possible to use

f3d --animation-index=1,2,3

[mwestphal]

Some discussions on discord:

• Small improvements needed on VTK: https://gitlab.kitware.com/vtk/vtk/-/merge_requests/12100
• If -1 is deprecated, there must be a way to recover the number of animations via an API

[mwestphal]

Fixed by #841