Fix sign errors in ϕ_kerr_schild function calculations

Author: dominic-chang

src/schemes/RayTrace.jl

@@ -194,12 +194,12 @@ function ϕ_kerr_schild(metric::Kerr{T}, rBL, ϕBL) where {T}
 
     term1 = a / (2temp) * log(abs(num / den))
     term2 = -atan(a, rBL)
-    ans = ϕBL - term1 - term2
+    ans = ϕBL + term1 + term2
     if isinf(ans)
         @warn "ϕ_kerr_schild is inf at rs=$rBL. This usually happens if the ray intersects the horizon."
-        return ϕBL - term2
+        return ϕBL + term2
     end
-    return ϕBL - term1 - term2
+    return ans
 end