controllers: Unify data structures for gains and parameters

[knmcguire]

We should unify the controllers to be more similar to the recent changes to the mellinger controller as in #1180

This way we can make multiple instances of controller python bindings of at least the PID controller as well, but the PID controller code is more spread out. Perhaps we should start with a list of which files those include

[knmcguire]

These are the three files to take into mind:

• https://github.com/bitcraze/crazyflie-firmware/blob/master/src/modules/src/controller_pid.c
• https://github.com/bitcraze/crazyflie-firmware/blob/master/src/modules/src/position_controller_pid.c
• https://github.com/bitcraze/crazyflie-firmware/blob/master/src/modules/src/attitude_pid_controller.c

There are actually already structs for a lot of gains, but they are not written in the same way. Moreover, the gains for attitude_pid_controller are initialized in attitude_PID.c and those of position are initialized in position_controller_pid. All these gains and variables need to be initialized outside of those files, so in controller_pid, together with all the parameters that sets them.