Investigate and design data model conversion from OSRD to NGE

[emersion]

Description and goal

First part of #7538

Look at both data models, write up a plan to convert the info, so that we're ready to start writing the code.

@louisgreiner has a small experimental Python script for this IIRC.

Acceptance criteria

A small document describing our strategy for the conversion.

[emersion]

Link to the Python script: https://github.com/osrd-project/osrd-nge