Use linearly increasing ramp as inflow BC

[BenjaminRodenberg]

Use a linearly ramp as initial condition: The inflow velocity starts for $$t=0$$ at $$u_x=0$$ and linearly increases to $$u_x=10$$ at $$t=1$$. For $$t>1$$ we use a constant inflow velocity of $$u_x=10$$.

Todos:

• [ ] Make sure that the state of the systemtests is clarified before merging #620, since it will influence the reference results.

Checklist:

• [ ] I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• [ ] I will remember to squash-and-merge, providing a useful summary of the changes of this PR.