add rails2sections

docs/lib3x-rails2sections.md

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+# rails2sections
+
+It returns all cross-sections described by rails. Combined with the `sweep` module, you can describe a more complex model.
+
+Rails should be indexed count-clockwisely.
+
+## Parameters
+
+- `rails` : A list of rails used to describe the surface of the model.
+
+## Examples
+
+	use <rails2sections.scad>;
+	use <hull_polyline3d.scad>;
+	use <sweep.scad>;
+	
+	rails = [
+	    [[5, 0, 5], [15, 10, 10], [25, 20, 5]],
+	    [[-5, 0, 5], [-15, 10, 10], [-25, 20, 5]],
+	    [[-5, 0, -5], [-15, 10, -10], [-25, 20, -5]],  
+	    [[5, 0, -5], [15, 10, -10], [25, 20, -5]]
+	];
+	
+	sections = rails2sections(rails);
+	
+	sweep(sections);
+	
+	#for(path = rails) {
+	    hull_polyline3d(path, 0.5);
+	}
+
+![rails2sections](images/lib3x-rails2sections-1.JPG)
+
+	use <bezier_curve.scad>;
+	use <rails2sections.scad>;
+	use <hull_polyline3d.scad>;
+	use <sweep.scad>;
+	
+	t_step = 0.05;
+	
+	rails = [
+	    bezier_curve(t_step, 
+	        [[1.25, 0, 5], [5, 20, 5], [16, 20, -2], [18, 20, 10], [30, 15, 8]]
+	    ),
+	    bezier_curve(t_step, 
+	        [[-1.25, 0, 5], [0, 20, 5],  [16, 22, -2], [18, 20, 10], [30, 25, 8]]
+	    ),
+	    bezier_curve(t_step, 
+	        [[-1.25, 0, -5], [0, 20, -5], [16, 20, 1], [18, 27, -3], [20, 27, -5]]
+	    ),
+	    bezier_curve(t_step, 
+	        [[1.25, 0, -5], [5, 20, -5], [16, 20, 1], [18, 17.5, -3], [20, 17.5, -5]]
+	    )
+	];
+	
+	
+	sections = rails2sections(rails);
+	
+	sweep(sections);
+	
+	#for(path = rails) {
+	    hull_polyline3d(path, 0.5);
+	}
+
+![rails2sections](images/lib3x-rails2sections-2.JPG)

src/rails2sections.scad

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+/**
+* rails2sections.scad
+*
+* @copyright Justin Lin, 2021
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib3x-rails2sections.html
+*
+**/
+
+function rails2sections(rails) =
+    let(
+        leng_rail = len(rails[0]),
+        leng_rails = len(rails)
+    )
+    [
+        for(i = 0; i < leng_rail; i = i + 1)
+            [
+                for(j = 0; j < leng_rails; j = j + 1)
+                    rails[j][i]
+            ] 
+    ];