Add cruise gCO2 calculator

[FatemaBohra]

Adding a cruise gCO2 calculator, to pair with the ferry and flight calculators.

I've added the relevant sources as a JSON, and a test in the tester.py file.

[J0ANMM]

Thanks for the proposal @FatemaBohra and apologies for taking so long to reply.

I am concerned about changing the input in def estimate_distance_from_duration of carbon_ferry.py (see here ), mainly for two reasons:

• For ferries, it seems more natural to provide the time in hours, since most people taking ferries are in trips of less than one day. As it is good practice to avoid decimals, the function uses minutes as input. Converting hours (or days) into minutes is straightforward.
• This change would break the calculator at Travelinho, where the interface shows the input in hours and it is converted into minutes to avoid decimals in the calculations.

I understand that for cruises an input in days seems more appropriate. Does it make sense keeping def estimate_distance_from_duration as is and creating an additional function for cruises that takes input in days and converts it into minutes before retrieving def estimate_distance_from_duration?