Bad performance in xmap generation

Enrico noticed that the xmap generation in ICON (ICON<-->HAMOCC) took very long. After some investigation, he found out that the changing the ordering of the indices in the idxlists significantly improved it.

This is a reproducer for this issue: reproducer.f90

$ mpif90 reproducer.f90 -I ../../../bin/yaxt/include/ -L ../../../bin/yaxt/lib/ -lyaxt -lyaxt_c -o reproducer
$ mpirun -n 4 ./reproducer
 xmap (bad):   0.98948631399999998     
 xmap (opt):    1.2026120000000473E-003