Move all global variable declarations to the appropriate subsystems

As it makes the subsystems code easier to understand.
Declare the most used variables in main sketch file as forward declarations.
Make all control input zero by default (except controlMode).
Minor changes.

flix/estimate.ino

@@ -8,6 +8,10 @@
 #include "lpf.h"
 #include "util.h"
 
+Vector rates; // estimated angular rates, rad/s
+Quaternion attitude; // estimated attitude
+bool landed;
+
 float accWeight = 0.003;
 LowPassFilter<Vector> ratesFilter(0.2); // cutoff frequency ~ 40 Hz