results are not consistent
==========================
  results differ between different systems, depending on CPU and compiler

  by now all calculations are done with double precision floating point,
  but still results are not consistent

  the code base is large, making it difficult to debug

  for debugging, two different systems are needed that create different
  results with identical input

  functions causing problems are
  compute() in 6s.cpp
    particularly the loop over response values per wavelength
  kernel() in computations.cpp
    input/output array bp is not initialized
    in the last loop, sbp must be > 0
    the second last loop after "if(k != ip)" needs a closer look
  [add more functions here]

  the code base needs to be compared against the latest 6S code
    requires comparing fortran with cpp
