HAL_AVR: constrain Vcc used in voltage_average()

this ensures a bad Vcc reading doesn't throw off analog inputs too
much
AC_WPNav: optimise to reduce memory copies

save a few cycles

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
AP_Buffer: instantiate as a .cpp file, and optimise for const usage
AP_InertialNav: reduce number of multiplies
AP_InertialNav: make some more functions const
AP_Param: make things a bit more efficient
AP_InertialNav: optimise some multiplies
Compass: use const references for some functions
AP_Buffer: remove bool return from add() method
AP_AHRS: use const references where possible
Copter: use the right constrain() type in Attitude code

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
Copter: use a reference to DCM matrix
AP_Motors: use the right constrain() function
Copter: more constrain fixes
Copter: simplify roll-pitch stabilize controller

remove check of whether to freeze i term when attitude is less than 5
degrees from horizontal.  I terms have been moved to the rate
controllers.
Copter: use DEGX100 constant for position_vector calcs
INS: add calibrated check

Checks is the scaling vector has been updated.  Required for pre-arm
checks for ArduCopter
Copter: add pre-arm check of radio and accel

Only checks throttle channel and accelerometer scaling
DCM: fixed airspeed bug

we incorrectly went via the wind estimate when we had a working airspeed sensor
DCM: fixed formatting