Allow implicit conversion from Linear to Affine

This will allow LinearTransforms to participate in AffineTransform
operator overloading with the appropriate promotion.

include/lsst/afw/geom/AffineTransform.h

@@ -109,7 +109,7 @@ class AffineTransform {
     ) : _linear(linear), _translation(translation) {}
 
     /** Construct an AffineTransform from a LinearTransform. */
-    explicit AffineTransform(LinearTransform const & linear)
+    AffineTransform(LinearTransform const & linear)
       : _linear(linear), _translation() {}
 
     /** Construct a translation-only AffineTransform from an Extent2D. */