Add missing Time Evolution Tutorials

[christian512]

This adds more tutorials on steady state and stochastic solver.

[hodgestar]

I ended up doing a reasonably thorough review because I got interested in the optomechanical example after this morning's discussion. :)

[christian512]

Thanks for the review! :) I updated the tutorial accordingly.

I also reduced the number of solvers used in optomechanical-steadystate to have a more reasonable runtime.

[christian512]

I have moved the tutorials into the time-evolution folder, which is required later on for creating the index page.I also updated the titles of the notebooks to match the titles at the tutorials website.

[Ericgig]

It would be nice if this example should be using the class interface instead of the function call in v5.
The class interface is to be used when computing the same system with different parameters, which is what is done here.

[christian512]

Thanks for the review @Ericgig! I changed from using brmesolve to BRSolver in the last commit. BRSolver currently requires that the spectra for the a_ops are given as coefficient. Hence, I can not pass a string spectrum.

I think it would be great if BRSolver also accepts other types of spectra.

[hodgestar]

Also looks good. Some questions:

• Are notebooks 15, 16 and 17 in a separate PR?
• It would be nice to automatically trim empty python cells at the end of the notebooks, or at least to warn about them in the test runs, but maybe just create an issue for that?

[christian512]

Thanks for the review, @hodgestar!
The notebooks 15, 16, 17 are the Stochastic Solver notebooks in #30.

I opened an issue to create a test for empty code cells in #38.

[hodgestar]

It looks like some images here are still in a top-level images folder, but once they're moved closer to their notebooks I'm happy for this to be merged.