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