gpx2energy: enhancements around the Car class

- use weight instead of mass
- cda as a property so that overriding cx actually works
- max speed equation as floats
gpx2energy: efficiency should decrease regen power, not increase it.
gpx2energy: show car data entered by the user, not the default ones.
gpx2energy: added regen efficiency input, fixed energy calculations.

Removed double efficiency multiplication for regen energy.