refactor calculations concept

[codingfabi]

Rethink and refactor calculation architecture:

• Streamline all different functions of calculations into a more clear hierarchy of calculation functions. Introduce calculate_co2_trip as basic function that serves as an abstract level on transportation mode calculations.
• Include main calculation functions on root level and maintain enhanced functions (commuting, trips) in separate directory/submodules
• Provide a way to add a custom emission factor for trips (optimal to do this via the configs object). I think we should handle this in a way to NOT look up an emission factor if the options object contains a custom_emission_factor entry.