Install ffmpeg on macOS M1

Using methods from imageio-ffmpeg on macOS M1 leads to the following error:

RuntimeError: No FFmpeg exe could be found. Install FFmpeg on your system, or set the IMAGEIO_FFMPEG_EXE environment variable.

According to this issue on github, the package FFmpeg is currently not included in the wheel of the imageio-ffmpeg library for macOS M1. But it would work using conda.

This would mean that we have to find a workaround to install ffmpeg if we don't want to use conda or wait that the maintainers of imageio-ffmpeg to create a proper wheel for this OS