Fixes + Unfinished skeletons export

[jeandet]

• Prints timepoints with ns resolution instead of us
• handles tt2000 special values
• explicitly support setting creating variable values from Python List
• makes creating attributes like creating variables
• introduces some Python dict export (skeletons WIP)
• exposes attributes values types

[jeandet]

@rodpib @brenard-irap @KatraB we can discuss here a bit about some new simpler CDF skeleton format which is user/JSON/YAML friendly.