Enforce fixed constraints recursively when setting pose on freegroups

[azeey]

Suggestion for #632

Testing the transform tool on three models attached with a detachable joint shows the problem. Without this change, we observe the previous rotation behavior.

detachable_joint_three_boxes.sdf

```xml true 0 0 10 0 0 0 1 1 1 1 0.5 0.5 0.5 1 1000 0.9 0.01 0.001 -0.5 0.1 -0.9 true 0 0 1 0 0 1 100 100 0.8 0.8 0.8 1 0.8 0.8 0.8 1 0.8 0.8 0.8 1 0 2 3.0 0 0.0 0.0 1 1 0.5 1 1 0.5 1 0 0 1 1 0 0 1 1 0 0 1 body M2 body2 3.3 2 3.0 0 0.0 0 1 1 0.5 1 1 0.5 0 1 0 1 0 1 0 1 1 0 0 1 body2 M3 body3 8.3 2 3.0 0 0.0 0 1 1 0.5 1 1 0.5 0 1 0 1 0 1 0 1 1 0 0 1 ```