Improve setParam/getParam behavior

[whoenig]

Currently, the following does not work as expected:

def test_set_cf_param_broadcast(self):
         swarm.allcfs.setParam('stabilizer/controller', 2)
         for cf in swarm.allcfs.crazyflies:
             assert cf.getParam('stabilizer/controller') == 2
         swarm.allcfs.setParam('stabilizer/controller', 1)

There are two issues that should be fixed:

• allcfs.setParams should update the cache of all Crazyflies that use the same channel. It might be best to do this directly in the C++ layer, and not the Python layer.
• Ideally, getParam() would have an option to pull the data from the CF, rather than just looking in the ROS parameter server for some (potentially stale) value.