Reimplement managed raster class in C++

[emlys]

Description

This PR reimplements the ManagedRaster class in C++ for better performance. Tested with sdr.calculate_sediment_deposition and the SDR sample data, it's now just slightly faster than the original implementation on main (0.59 vs 0.61 seconds), whereas the python implementation on feature/routing-refactor takes nearly twice as long.

I'm a beginner at C++ so I'm sure I'm not always using the right conventions or design patterns, but this seems like an okay starting point. I took some guidance from the Google C++ style guide. Ignoring some guidelines that don't seem to make sense in this context. For instance, I have all the code in a header file because splitting it out into ManagedRaster.h and ManagedRaster.cpp significantly slowed it down.

Happy to discuss and walk through this PR on a zoom call as well.

Checklist

• [ ] Updated HISTORY.rst and link to any relevant issue (if these changes are user-facing)
• [ ] Updated the user's guide (if needed)
• [ ] Tested the Workbench UI (if relevant)

[emlys]

@phargogh this is finally ready for re-review! The biggest change is I figured out how to implement the iterators properly, so they can be iterated with the usual syntax, solving the issues with tracking/incrementing values properly.