Add documentation and very simple test for array atan(), part 2
(files I forget in the previous commit).
diff --git a/test/array.cpp b/test/array.cpp
index 5f49fc1..010fead 100644
--- a/test/array.cpp
+++ b/test/array.cpp
@@ -178,6 +178,7 @@
   VERIFY_IS_APPROX(m1.asin(), asin(m1));
   VERIFY_IS_APPROX(m1.acos(), acos(m1));
   VERIFY_IS_APPROX(m1.tan(), tan(m1));
+  VERIFY_IS_APPROX(m1.atan(), atan(m1));
   
   VERIFY_IS_APPROX(cos(m1+RealScalar(3)*m2), cos((m1+RealScalar(3)*m2).eval()));