Use geodetic distance instead of number of pixels for radius

[vlandau]

Allow radius for moving window to be specified in meters instead of number of pixels when inputs are in GCS WGS 84 (decimal degrees).

Will depend on lower corner coordinates of raster as well as cell size, and use the equation to calculate geodetic distance between two pairs of decimal degrees. Should still be able to do this using a comprehension in Julia.

[vlandau]

This would only be advisable is source strength is also adjusted by pixel area