Add core_adapter

Core adapters allow cores to be attached to daylight spool holders.

models/core_adapter/core_adapter.scad

@@ -0,0 +1,13 @@
+module core_adapter () {
+    $fn = 100;
+	difference () {
+		union () {
+			cylinder(r1 = 13.2, r2 = 12.8, h = 16.6, center = true);
+			translate([0, 0, -8.8]) cylinder(r = 15, h = 1, center = true);
+			translate([14.5, 0, -.5]) cube([3.6, 3.6, 17.6], center = true);
+		}
+		cube([8.3, 8.3, 21.5], center = true);
+	}
+}
+
+core_adapter();