Add m_per_s() to units.Velocity (#495)

Author: element15

skyfield/units.py

@@ -133,6 +133,10 @@ def __init__(self, au_per_d=None, km_per_s=None):
     def km_per_s(self):
         return self.au_per_d * AU_KM / DAY_S
 
+    @reify
+    def m_per_s(self):
+        return self.au_per_d * AU_M / DAY_S
+
     @reify
     def AU_per_d(self):
         if not Velocity._warned: