Acoustic Integrator rework

[tjueterb]

This PR

• Adds an energy loss stopping criterion to all acoustic integrators
• Implements several small bugfixes for all acoustic integrators
• Adds tests for all acoustic integrators

Energy loss stopping criterion.

Stopping after a certain path depth is usually not really helpful in acoustics. Stopping after a certain energy loss (e.g. 65 dB if the goal is estimating T60) leads to the desired path lengths while at the same time not tracing paths with little energy unnecessarily long.

Russian Roulette

A simple stopping criterion without russian roulette techniques introduces bias (the ETC drops off afterwards).
This is usually not a problem.
Do we want to implement russian roulette anyways?

Testing

Checklist

• Implement for all integrators
• Add russian roulette logic?
• My code follows the style guidelines of this project
• My changes generate no new warnings
• My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• I have commented my code
• I have made corresponding changes to the documentation
• I have added my changes to docs/release-notes.rst
• I have added tests that prove my fix is effective or that my feature works
• I cleaned the commit history and removed any "Merge" commits
• I give permission that the misuka project may redistribute my contributions under the terms of its license