Fix negative linear weights

[giacrossi]

In case of WENO interpolation, linear weights (kappa objects in WenOOF) can assume negative values: this behaviour can lead to unstable schemes and therefore a suitable strategy to avoid this must be implemented. In particular, the strategy adopted in A Technique of Treating Negative Weights in WENO Schemes, Shi, J., Hu, C., Shu, C., JCP, 2002, vol. 175, pp. 108-127, doi:10.1006/jcph.2001.6892 seems very intresting and efficient.