Accessibility improvement

[ciupava]

• multimodal trips
• generating ttm valid for future scenarios ( Generating 'empty' destination points #20 )
• consider multi-time trips (IE different time of the year/day, median/average)

[ciupava]

dealing with multimodal trips in traveltime_multimodal.ipynb

[martinfleis]

This is not all implemented, correct @ciupava? So we can keep it open for potential follow-up.