[pretel] optimise algorithm of generate_atm.py routine

This issue is specific to the optimisation of the generate_atm routine. The problem with the current implementation is that it interpolates from the ATM file to the GEO file at every time step.

assigned to @saillour.thomas

mentioned in merge request !179 (closed)

added lang:python type:enhancement labels

mentioned in merge request !204 (merged)

closed with merge request !204 (merged)

mentioned in commit 2b24cd57