[transport-mode] Version 2.0 of transport-mode

[marioscrock]

Changes to be discussed are mentioned below.

Introduce hierarchy for specific transport means (cf. #27)

• Add generic "Rail" value? Or "Train"?
• Keep "Local-regional/long-distance" distinction? Use spatial coverage instead?

Simplify IRIs and deprecate old values

• Keep together "Tram, Light rail"?
• Simplify "Metro, Subway train" and improve description.

Add your comments or propose additional changes to be included in the new release.