Changed the parameter struct in modular_structs to have an array of PID_t types

- added some comments in commands.c/h about usage of the file
- added a MAX_CONTROLLER_ID and MAX_CONTROL_VAL_ID to the enums in commands.h
- implemented cb_set_control in callbacks.c to work with the new parameter struct
- changes the parameter_struct to now contain an array of pid_controllers instead of a bunch of random PID_t members
- the new parameter_struct array can now be accessed by controller ID as defined in commands.h