Add fractal maps as null models

[alexgenin]

Fractal maps allow including some autocorrelation (at a known level) in the landscape. This is ideal e.g. to provide a more stringent null model where some autocorrelation is kept in the landscape, instead of randomizing everything (i.e. no autocorrelation). Algorithm is provided by ref below (I am attaching an extract from the relevant page).

Turner and Gardner Landscape Ecology in Practice 2015. Springer