added polytransversals

docs/lib-polytransversals.md

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+# polytransversals
+
+Crosscutting a polyline at different points gets several transversals. This module can operate reversely. It uses transversals to construct a polyline. For example, imagine that you have the following transversals:
+
+![polytransversals](images/lib-polytransversals-1.JPG)
+
+This module can use them to construct the polyline:
+
+![polytransversals](images/lib-polytransversals-2.JPG)
+
+When using this module, you should use points to represent each transversal. 
+
+You can view it as a better polyline2d module. If you have only the points of a path, using `polyline2d` or `hull_polyline2d` is a simple solution. If you know the transversals along a path, you can use `polytransversals` to get a better polyline.
+
+## Parameters
+
+- `transversals` : A list of transversals. Each transversal is represented by a list of points. See the example below.
+
+## Examples
+
+	include <rotate_p.scad>;
+	include <polytransversals.scad>;
+	include <hull_polyline2d.scad>;
+	
+	r = 35;    
+	tran_pts = [[-5, 0], [0, 5], [5, 0]];
+	
+	trans = [
+	    for(a = [0:10:120])
+	        [
+	            for(p = tran_pts) 
+	                rotate_p(p, [0, 0, a]) + [r * cos(a), r * sin(a)]
+	        ]
+	];
+	
+	polytransversals(trans);
+	
+	#for(tran = trans) {
+	    hull_polyline2d(tran, 0.5);
+	}
+
+![polytransversals](images/lib-polytransversals-3.JPG)