Addressing viewshed numerical error issue on M1.

RE:natcap#1562

src/natcap/invest/scenic_quality/viewshed.pyx

@@ -815,8 +815,21 @@ def viewshed(dem_raster_path_band,
             if target_distance > max_visible_radius:
                 break
 
-            z = (((previous_height-r_v)/slope_distance) *
-                 target_distance + r_v)
+            # This is a weird platform-specific workaround addressing
+            # https://github.com/natcap/invest/issues/1562
+            # On M1 macs, the all-in-one-line addition of _product and r_v
+            # would create small but noticeable numerical error.  Breaking the
+            # calculation onto two lines eliminates the numerical error.  This
+            # behavior is reproducible in C, outside of Cython on an M1 mac.
+            # So, this calculation would introduce error:
+            #   z = (((previous_height-r_v)/slope_distance) * target_distance) + r_v
+            # while the formlation below does not.
+            #
+            # Some of this may be related to the fact that x86 chips have
+            # extended precision for FPU-based calculations while M1 ARM chips
+            # do not.  Still, that doesn't explain why the error is introduced.
+            _product = (((previous_height-r_v)/slope_distance) * target_distance)
+            z = _product + r_v
 
             # add on refractivity/curvature-of-earth calculations.
             adjustment = 0.0  # increase in required height due to curvature