Topic 4

# Navier-Stokes equation -- flow around cylinder benchmark

- set up more complicated domain and generate mesh
- using mixed finite element spaces for coupled systems
- solving nonlinear problems using `NonlinearVariationalProblem` and `NonlinearVariationalSolver` classes
- reproduce the [flow around cylinder benchmark](http://www.featflow.de/en/benchmarks/cfdbenchmarking/flow/dfg_benchmark1_re20.html)