fixes #795 Unetary support for vectors

Author: jf---

CHANGELOG.md

@@ -10,6 +10,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+* Added a `invert` and `inverted` method `compas.geometry.Vector`.
+* Added unetary `__neg__` operator for `compas.geometry.Vector`
+
 ### Changed
 
 ### Removed

src/compas/geometry/primitives/vector.py

@@ -255,6 +255,9 @@ def __pow__(self, n):
         """
         return Vector(self.x ** n, self.y ** n, self.z ** n)
 
+    def __neg__(self):
+        return self.scaled(-1.0)
+
     def __iadd__(self, other):
         """Add the components of the other vector to this vector.
 
@@ -676,6 +679,41 @@ def unitized(self):
         v.unitize()
         return v
 
+    def invert(self):
+        """Invert the direction of this vector
+
+        Note
+        ____
+        a negation of a vector is similar to inverting a vector
+
+        Examples
+        --------
+        >>> u = Vector(1.0, 0.0, 0.0)
+        >>> v = u.copy()
+        >>> u.invert()
+        >>> u == v
+        False
+        >>> u.invert()
+        >>> u == v
+        True
+        >>> v == --v
+        True
+        """
+        self.scale(-1.)
+
+    def inverted(self):
+        """Returns a inverted copy of this vector
+
+        Examples
+        --------
+        >>> u = Vector(1.0, 0.0, 0.0)
+        >>> v = u.inverted()
+        >>> w = u + v
+        >>> w.length
+        0.0
+        """
+        return self.scaled(-1.)
+
     def scale(self, n):
         """Scale this vector by a factor n.