roads 1.2.0

• Change dynamic least cost paths (DLCP) to iterative least cost paths (ILCP) throughout
• Change default roadMethod to "ilcp" in projectRoads
• Add ability to use a custom weightFunction and add a weightFunction gradePenaltyFn
  that determines the grade between two cells
• Add a vignette and a demo data set dem_example to show how gradePenaltyFn can be used.
• Change argument name from cost to weightRaster since it no longer represents a cost
  surface and can now be inputs to the weightFunction. Also change roadsInCost to roadsInWeight.
• returned roads are no longer unioned together.
• Deprecate getDistFromSource and use terra::distance instead.
• Fix bug in getLandingsFromTarget and change so that patches are used for 0,1 rasters and
  ids are used otherwise using terra::as.polygons to make it faster.