make units of mt_speed, mt_distance by default SI units?

Hi @bartk, translating the animove scripts, and seeing my self adding in dozens of lines the set_units(....) to ensure that the units are m/s or m for the following functions, which not all "understand" units, I was thinking if it might make sense to have default units for these 2 functions. Because I guess that is what most people work with, and of course one can change it if needed.