Allow setting param variable from within the firmware (ie. For apps)

[ataffanel]

Allowing apps to write param variables at runtime would be very helpful as it will gie apps the same level of control on params that script communicating over the radio to the Crazyflie have.

The param subsystem design assumes that params are never changed by the firmware after initialization.The python crazyflie lib is design with that assumption and cache all parameter values at connection, there is no mechanism in place to update this cache if a variable changes in the firmware.

There is already functions implemented in the firmware to read and write param variables. These functions should be changed to notify the client that a variable has been changed by sending a CRTP packet.

Threre is also going to be some change needed in the python lib and maybe in the client. Tickets for these changes will refer to this ticker.

Related to #502.