AP_InertialSensor: pass/return Vector3f by const ref instead of by value

apm2-quad: saves ~500byte
AP_InertialSensor: removed assignment that had no effect, replaced
temporary objects with references, moved object declarations
--> saves some space :)
DataFlash: const references ... elide copies
Copter: replace objects with const refs ... saves space ...
AP_InertialSensor: remove (previously added) const ref to remove
shadowing warning
AP_InertialSensor: add consts & virtual specifier to overriding subclass
methods
AP_HAL_SMACCM: rename function parameter

in c++11 the identifier "override" has a special meaning. in order write
future-proof code, "#define override" is needed. this clearly conflicts
with any occurrences of "override" in the code.
therefore it's renamed.
AP_HAL_PX4: rename function parameter

in c++11 the identifier "override" has a special meaning. in order write
future-proof code, "#define override" is needed. this clearly conflicts
with any occurrences of "override" in the code.
therefore it's renamed.
AP_HAL_Empty: rename function parameter

in c++11 the identifier "override" has a special meaning. in order write
future-proof code, "#define override" is needed. this clearly conflicts
with any occurrences of "override" in the code.
therefore it's renamed.
AP_HAL_AVR_SITL: rename function parameter

in c++11 the identifier "override" has a special meaning. in order write
future-proof code, "#define override" is needed. this clearly conflicts
with any occurrences of "override" in the code.
therefore it's renamed.
AP_HAL_AVR: rename function parameter

in c++11 the identifier "override" has a special meaning. in order write
future-proof code, "#define override" is needed. this clearly conflicts
with any occurrences of "override" in the code.
therefore it's renamed.
AP_HAL: add c++11 "override" identifier to macros
AP_AHRS: add virtual & c++11 override specifiers
Copter: use const refs instead of unneeded copies
AP_InertialSensor: add c++11 override specifier
AP_Baro: add c++11 override specifier to virtual functions in subclasses
Filter: add c++11 override specifier to overriding virtual functions in
subclasses