Support for Distance-Cost Raster in Dispersal

[katarzynam-165]

The dispersal calculation in initialise() currently assumes uniform distance costs across the landscape. A feature allowing distance-cost rasters would enable users to model barriers, like mountains, that restrict dispersal differently than open areas. This enhancement would improve model realism for species dispersal and invasion patterns over time.

Proposed Solution:

1. Allow users to provide a distance-cost raster to set custom dispersal costs.
2. Alternatively, add a "distance multiplier" raster option to scale the calculated distances by the raster values.