Inverted nested loop

This issue will be used to track the steps and progress towards testing a parallel implementation of the function tra_adv_fct taking the outer tracers loop and making it the innermost nested loop in all the calculations.

This will make the number of OMP calls stay at the same number and not grow even when more than 1 tracers are being calculated.

It will also multiply the memory required for this function by the number of tracers even when not using OMP, as we will now be doing all the calculations at the same time, instead of one after another. Then, the memory requirements won't change when using OMP.