Increase mass of base

Makes tipping behavior in simulation more plausible
I assume that the mass of an assembled base is at least this heavy, given that the robot is around 50lbs total

stretch_description/urdf/stretch_main.xacro

@@ -13,7 +13,7 @@
         xyz="-0.109461304328163 -0.000741018909047708 0.0914915269429946"
         rpy="0 0 0" />
       <mass
-        value="1.0723782659782" />
+        value="6.0" />
       <inertia
         ixx="0.00310580907710135"
         ixy="1.5182848191076E-06"