Make reference Mars PCM start/def bundles available

[emillour]

We already provide "initial conditions" but in the form of start_archive.nc files from MCD(v5.3 or v6.1) runs.
Most people don't find this convenient and would prefer to have bundles (or "bench" configurations) containing, for a given setup, the start and def files to use. e.g.:

• a complete MCDv6.1 (thermosphere+chemistry+ions) 64x48x73 setup
• a complete MCDv5.3 (thermosphere+chemistry+ions) 64x48x49 setup (see #151)
• a GCM6 "lower atmosphere" (water+dust+co2 cycle) 64x48x54 setup
• some older GCM5 "lower atmosphere" (water+dust+co2 cycle) 64x48x32 setup?
• a GCM5.5 (still to de determined, but probably GCM5-like dust and vertical resolution) "lower atmosphere" 64x48x26 setup?
• etc.

[emillour]

Partially done with the additions in ​https://web.lmd.jussieu.fr/~lmdz/planets/mars/ of

• reference_GCM5_64x48x32.tar
• reference_GCM6_64x48x54.tar

[emillour]

Finalized concerning GCM6 since (~june 2024) with the addition of

• reference_GCM6_64x48x54.tar
• reference_GCM6_64x48x73.tar