Mitransient

Refactors `mitransient` so that it's compatible with Mitsuba 3.6.X / Dr.Jit 1.0.X. This brings a (pretty noticeable) improvement in efficiency/execution time :)

How to upgrade:
* If you installed Mitsuba with pip, you can just `pip install -U mitsuba==3.6.4`. If you compiled Mitsuba from source, you will need to pull and recompile.
* If you're using [`y-tal`](https://github.com/diegoroyo/tal) you will need to update to `>=0.19.0` (`pip install -U y-tal==0.19.0`)

Notes:
* Some parts have not been ported to Mitsuba 3.6.X yet. This includes spatial and temporal filtering of the renders, and the `transient_prbvolpath` integrator (i.e. participative media). We plan to also port these in a future update.
* If you need these for your work, or if you find any other issue, feel free to contact us.

* 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`.