Delete the experimental functions calculate_artificial_damping()

[ufechner7]

It is not well defined what this functions should do. Better delete them now, we can add them later when clear requirements and tests for these functions are available.

[ufechner7]

This would increase the test coverage.

[1-Bart-1]

Please don't remove the smooth_circulation function as it is used in gamma_loop and in the stall_model example.
Feel free to remove calculate_artificial_damping() as it is not used anywhere.
It is quite clear from the plots from the stall_model example what the effect of smooth_circulation is.