Added curves.scad, includes helix_curve() ATM

curves.scad

@@ -0,0 +1,17 @@
+include <constants.scad>
+use <math.scad>
+
+
+/* A circular helix of radius a and pitch 2πb is described by the following parametrisation:
+x(t) = a*cos(t),
+y(t) = a*sin(t),
+z(t) = b*t
+*/
+
+
+function b(pitch) = pitch/(TAU);
+function t(pitch, z) = z/b(pitch);
+
+function helix_curve(pitch, radius, z) =
+    [radius*cos(deg(t(pitch, z))), radius*sin(deg(t(pitch, z))), z];
+