[WIP] Consistent mass fluxes between bulk flow and scalar solvers

[bigfooted]

Proposed Changes

Give a brief overview of your contribution here in a few sentences. We need a different treatment of the convective scalar fluxes to improve results for the species solver. This PR deals with the specific case of the incompressible species solver.

Related Work

Resolve any issues (bug fix or feature request), note any related PRs, or mention interactions with the work of others, if any. connected to #726 and #721, and discussion #1695

my checklist:

[] check residual convergence for old and new testcase (primitive venturi mixer) [] check convergence for turbulence problems, still problematic? Should we implement upwind scheme for species only (for now)? [] check implementation of inlet/outlet bc [] compressible flow vs incompressible flow []

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.
• [ ] 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.