Mitransient

* Improved tonemapping and video generation
* `transient_volpath` integrator is more efficient now
* New `mitr.cornell_box()` function which provides a minimal scene with a time-resolved integrator, useful for demonstrations
* New `mitr.vis.rainbow_visualization(...)` which shows the rendered scene with a rainbow colormap that represents time of flight
* Added documentation and examples (see https://mitransient.readthedocs.io/en/latest/)

First complete public version of `mitransient`, with updates to the documentation.

First complete public version of `mitransient`.