feat(Blade): add option to change af interpolation

The current implementation of the airfoil interpolation leads to a very
high computational overhead, as the lift and drag coefficients need to
be retrieved for each element one at a time when each element has a
slightly different airfoil.

The "old way" of doing it (i.e. having the same airfoil until the next
reference section), allows for a much faster calculation of the lift and
drag coefficients as only the few reference airfoil polars need to be
polled for all matching elements at once. If the user inputs enough
airfoils, an accurate solution can still be acheived without too much
overhead.

Simple testing indicate that the complete interpolation leads to a
computing time up to 50x larger than without airfoil interpolation.