Add an axis aligned box in the geometry module.

Some naming questions:
- for "extend" we could also think of: "expand", "union", "add"
- same for "clamp": "crop", "intersect"
- same for "contains": "isInside", "intersect"
=> ah "intersect" is conflicting, so that eliminates this one !

Eigen/src/Geometry/Hyperplane.h

@@ -203,7 +203,7 @@ public:
     }
   }
 
-  /** \returns the transformation of \c *this by the transformation matrix \a mat.
+  /** Applies the transformation matrix \a mat to \c *this and returns a reference to \c *this.
     *
     * \param mat the Dim x Dim transformation matrix
     * \param traits specifies whether the matrix \a mat represents an Isometry
@@ -223,7 +223,7 @@ public:
     return *this;
   }
 
-  /** \returns the transformation of \c *this by the transformation \a t
+  /** Applies the transformation \a t to \c *this and returns a reference to \c *this.
     *
     * \param t the transformation of dimension Dim
     * \param traits specifies whether the transformation \a t represents an Isometry