Improve Py Segments handling with nanobind

[ferdonline]

Context

Improve handling of Python Segment objects leveraging nanobind

Scope

• Reimplement o2loc where a lot of Py refcount was being managed manually. Code got much simplified, without any manual ref count changing.
• Make Seg_init an empty stub, since it's never called

[bbpbuildbot]

Logfiles from GitLab pipeline #220112 (:no_entry:) have been uploaded here!

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[bbpbuildbot]

Logfiles from GitLab pipeline #220127 (:no_entry:) have been uploaded here!

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[azure-pipelines[bot]]

✔️ 1b20fae3760c0daf7b078c53ca38e034f5229299 -> Azure artifacts URL

[bbpbuildbot]

Logfiles from GitLab pipeline #220205 (:white_check_mark:) have been uploaded here!

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• :white_check_mark: spack_setup
• :white_check_mark: build:nmodl
• :white_check_mark: build:neuron:nmodl:intel:legacy
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• :white_check_mark: test:neuron:nmodl:nvhpc:omp

[sonarcloud[bot]]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 67.26%. Comparing base (12aba8b) to head (3d6ebb3).

Files	Patch %	Lines
src/nrnpython/nrnpy_nrn.cpp	77.77%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2954 +/- ## ========================================== + Coverage 67.25% 67.26% +0.01% ========================================== Files 571 571 Lines 104899 104879 -20 ========================================== - Hits 70553 70551 -2 + Misses 34346 34328 -18 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[bbpbuildbot]

Logfiles from GitLab pipeline #220249 (:white_check_mark:) have been uploaded here!

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• :white_check_mark: spack_setup
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• :white_check_mark: build:neuron:nmodl:intel:legacy
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[azure-pipelines[bot]]

✔️ 3d6ebb3cd95b7fc83625ee69e688c26c727b4fa5 -> Azure artifacts URL