Test build on win/vs2017-2019. Update findTBB to find oneTBB. Update tbb threads control (tbb::global_control) and loop (tbb::parallel_for_each) and mutex (std::mutex). Remove warnings.

Add Observer. Remove functions from python side and add as observer of Problem. Add loads directions as functions. Minor changes.

Add formulation classes to build set of equations, and remove build methods from Element (flow only).
Draft methods to compute freestream velocity.
Rework data structure of boundary.
Add const.
Improve test cases.