Inlcuding curvature length scale in listonwind, and fixing the calculation of curvature

This fixes the calculation of curvature by including the distance between points. It also implements the curvature length scale ETA in the ListonWind algorithm (see eta in Eq. 14 in Liston et al., 2006). To be specified in the configuration via:

VW::algorithms  = LISTON_WIND
VW::liston_wind::eta = 5000