implement combinations of two averages

[aymeric-spiga]

for instance zonal+diurnal averages

[aymeric-spiga]

actually zonal averages have to be combined with diurnal averages (unless case of same local time everywhere) and this is not the case : this has to be fixed

[aymeric-spiga]

maybe also add possibility to do global averages (pondering with areas)
and annual averages (Ls, per month pondered with month duration, and local time)