Clean LoadRestart for Flow/Turb/Heat

[TobiKattmann]

Hi,

this is just a small chore which cleans LoadRestart routines and strengthens the similarities between the function. Declare vars where needed (instead of the top of the func), make things const and some anticipation of clang-format stuff (without explicitly running it over that file)
This was originally done in #1388 but i dont want to pollute that PR with unrelated stuff

I also added a nVarTurb Variable which can be accessed via the Config. Might be handy in some places, instead of always going through the full enum class TURB_MODEL.

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.

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags, or simply --warnlevel=2 when using meson).
• My contribution is commented and consistent with SU2 style.
• 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.

[pcarruscag]

Thank you for this 👍