added from_date and to_date to airline alliance membership file

[wsteitz]

I added two new columns to the alliances membership file: from_date and to_date. This allows to get the alliance membership at a given date. There are still some airlines and dates missing, but I plan to do some more updates once we agreed on the file layout.

[da115115]

Thanks for that patch submission. Could you add yet another column, namely env_id (enveloppe ID)? When that (env_id) field:

• is empty, it means that the row corresponds to the current values.
• has got a positive integer value, it corresponds to an older version of the values.

So:

• When the row corresponds to the current values, put an empty field.
• When the membership details for a given airline:
  • Set an integer value to the env_id field, starting from 1. Set the date_to to the current date.
  • Add a new row with the new current values. Set the date_from to the current date minus one day.

[wsteitz]

Good idea, makes it easier to get the current alliance members from the file. I added the env_id as you suggested and committed the changes to https://github.com/wolfer7/optd/tree/alliances.