add spherical_coordinate

docs/lib3x-spherical_coordinate.md

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+# spherical_coordinate
+
+Converts from Cartesian to Polar coordinates. It returns `[radius, theta, phi]`.
+
+**Since:** 3.0
+
+## Parameters
+
+- `point` : The Cartesian coordinates of a point.
+
+## Examples
+
+    use <util/spherical_coordinate.scad>;
+    
+	coord = spherical_coordinate([100, 100, 100]);
+	r = round(coord[0]);
+	theta = round(coord[1]);
+	phi = round(coord[2]);
+    assert([r, theta, phi] ==  [173, 45, 35]);