[WIP] Update turbo interfaces

[joshkellyjak]

Proposed Changes

Update to the interface initialisation to enable both mixing plane and sliding interfaces within the same turbomachinery simulation. Addition of mass flow outlet to Giles BC.

Related Work

This issue solves a problem reported by a number of users, when simulating radial turbomachinery cases a second-order accurate solution cannot be achieved in some cases when using mixing plane interfaces. This is due to circumfrential averaging in regions where the flow exhibits large circumfrential nonuniformity and/or interspace gaps between adjacent blade-rows/zones small. This issue has been reported by @LorenzoFabris, in a presentation at the SU2 Conference 2022 "Performance Analysis and Shape Optimization of a Radial Compressor with the Open-Source Software SU2" and by myself.

Mass flow outlet included in the PR as requested by users.

TBC: -Update APU Turbocharger test case and regression test to include this feature, add diffuser and converge 2nd order.

PR Checklist

Put an X by all that apply. You can fill this out after submitting the PR. If you have any questions, don't hesitate to ask! We want to help. These are a guide for you to know what the reviewers will be looking for in your contribution.

• [X] I am submitting my contribution to the develop branch.
• [X] My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• [X] My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• [X] I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• [ ] I have added a test case that demonstrates my contribution, if necessary.
• [ ] I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.