Replace vapory with an alternative solution.

Despite vapory is a nice tool, it looks pretty abandoned. The issues are not addressed, the last release is in 2014.
The [request](https://github.com/Zulko/vapory/issues/57) for making a new release remains unanswered since October 2020.
Therefore, we should consider an alternative solution.

@csadorf pointed out a nice library called [Fresnel](https://fresnel.readthedocs.io/en/stable/). It looks very well maintained and, importantly, has all the features we are currently using for generating images with povray.

Obviously, we might also consider other tools that are available.