Refactor RxD module to use SoA data structures

[ctrl-z-9000-times]

This is an idea for refactoring the RxD module.

Currently the RxD module calculates the geometry of each segment:

• Segment surface area
• Segment volume
• Segment neighbor area (the cross-sectional area at the segment boundaries)

However, the RxD module has performance issues (#2950) ...
We should re-implement these three calculations in C++ using the new SoA data structures.

Notes:

1. Memory need not be allocated until the moment the RxD module requests the segment geometry.
2. Initialize the memory to an invalid value (such as -1) to indicate that the value has not yet been computed.
   Then only calculated the geometry for the segments that the RxD module requests.
3. Expose the segment geometry to the RxD module via private / undocumented python functions.
4. The SoA structures can easily accomidate this.
5. The risk of introducing bugs is relatively low since we can use the current implementation as a reference.

[nrnhines]

I very much like the idea. I fervently hope that src/neuron/container allows this to be straightforward. I don't know if there are pointers at the RxD interpreter level into the low level memory, but, if so, I also hope that DataHandle will nicely always be valid when low level memory is rearranged and reallocated. And that DataHandle will allow #1549, which has been long languishing since 2021, will finally be completable.