NEMO: protection for viscous flow with ionization

[CatarinaGarbacz]

Proposed Changes

adding basic protection for the case of a nonequilibrium viscous flow with ionization

Related Work

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 the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags, or simply --warnlevel=2 when using meson).
• [ x] My contribution is commented and consistent with SU2 style.
• [x ] I have added a test case that demonstrates my contribution, if necessary.
• [x ] I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.