Fix for multithread

cc: @gtribello

Some of the optimizations done in 5d0ff6aa
would break multithread calculations. In particular one should not use
non-const static variables. I made these fixes:

- for variables fd, C, and D, I use a std::array of fixed (exaggerated) size
- for other variables, unfortunately I just had to remove the static specifier

In case this slows down the code significantly, we should work out some alternate
solution. Maybe this would require changing the interfaces to some method so
that they accept e.g. a pointer rather than a vector. I have a tentative implementation
but did not have time to really test it so I prefer not to merge it.