Animate multiple plots on same axes

[JoelLucaAdams]

• Added support for animating multiple plots on the same axes. This change leads to a refactor of the animation setup code to a common function _generate_animation which is shared for both single and multiple animations.
• Changes doi for datasets to zenodo link as it falls over when when there are multiple releases of the same project.
• Updated 2D tutorial dataset to include poynting flux.
• Added new xarray.Dataset.epoch.animate_multiple accessor.

Note: I am very open to any changes to the newly refactored _generate_animation function as it is slowly starting to become unreadable.

Breaking changes

A moving window is no longer automatically detected and the user must specify move_window when animating. This is because when animating multiple plots on the same axes sometimes the user might want to remove data below a threshold such as the change seen in the final animation on the animations page of the documentation so that the pulse looks clean. If they did not trim the data then a "haze" of the lower end of the cmap specified for the topmost plot appears. Doing this trimming of data causes NaNs to be present in the data which previously tried to generate a moving window which was not desired.