Extending functional niche generator to include skew normal distribution

[tretherington]

We don't necessarily know that the fundamental niche is elliptical, and some of the early niche theory that virtualNicheR is based upon used more 'egg-shaped' niches (Maguire 1973).

Jiménez et al. (2019) generated fundamental niches using a skew normal distribution which does produce 'egg-shaped' niches, so this could be a useful future enhancement for virtualNicheR if there was a need for these shapes.