COS8 switched to Python3.8

The coefficients in the PHE had signs inconsistent with the direction of propagation,
inhereted by PyPropagate. For that reason the refractive index needed to be conjugated.
This fixes the behaviour.
See my manuskript for a derivation.

Interface zwischen Python und C++ verbessert, so dass weniger kopiert und
transponiert wird. Scheint ne Menge zu bringen :-)