Fix/3dh2d HelmSolve

This fixes a bug in 3DH2D where HelmSolve is being called with the wrong index for m_lines. This leads to wrong boundary conditions being imposed on the field.

The channel flow test was passing because in that case all boundary conditions are zero. Therefore, a test for the Couette flow (which fails without this fix) was included. Also, minor changes regarding indentation and duplicate code were made.