Bugfix at order 3 (hopefully was not affecting final results, thanks to Pablo...

Bugfix at order 3 (hopefully was not affecting final results, thanks to Pablo Winant for spotting this):
- in the 3rd derivatives matrix, among symmetric elements, the first one had
the right values but the following ones were set to zero
- moreover, the k-order DLL was trying to add all the symmetric elements in the
folded tensor, instead of only keeping one value among all the symmetric ones
- hopefully, Ondra's tensor library was (silently) refusing to add symmetric
elements after the first (and right) value had been added
- so the final result was correct