Proposed Changes

Cleanup CHeatSolver.cpp

• define loop vars in the loop
• make stuff const auto
• remove unnecessary vars - or move definition&allocation where needed
• some minor logical changes which hopefully are equivalent ( and therefore don't change the reg test 🤞 )
• Fix primal unsteady Heat restarts

@oleburghardt might be interested in these changes ;)

Can be seen as a follow up to #1256 of @maxaehle (although I dont specifically fix memory leaks)

PR Checklist

• 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.