Add methods that accept Raster as an input type

[vlandau]

May do this after implementing multi-processing in chunks.

Adding a method that accepts a Rasters.Raster as an input step might make folding Omniscape into larger geospatial workflows more seamless. To do this, I should probably create a base method that just accepts an AbstractArray with no metadata about projection, transform, etc. Then making new methods for additional data types will be pretty seamless and will involve only adding a few extra lines of code around a call to the base method to process inputs and format/construct outputs of the same type.