Feature/diff thrust curve for cw and ccw props

DJI snail motors use on HEX have different PWM->RPM coefficient (and thus PWM-> Thrust and PWM-> Torque) depending on spinning direction (e.g. see PWM commanded at hover on HX14). This PR allows to simulate this issue, by allowing to individually set the thrust/torque curve for clockwise and counterclockwise propellers. PWM at hover from experiment:

PWM at hover in snap_sim using this PR (parameters not included because specific for HX14)