AP_InertialNav: remove unused AP_InertialSensor pointer
Copter: add comment (unit of G_Dt)
AP_Baro: add time-unit comment
Filter: add consts
AC_Sprayer: add consts, update comments

* removed some comments that were left over from the classes this class
was copypasta'ed from
Copter: Log.pde: replace unused copy with ref (saves again some bytes)
AC_Sprayer: replace unnecessary object with const ref
AC_WPNav: replace unnecessary objects with const refs
AC_Sprayer: add doxygen info
Copter: use const refs instead of unneeded copies
AC_Fence: make member pointer to AP_InertialNav const, remove unused
gps-pointer member
AC_Fence: replace "_ina->geT_position().z" with more efficent
"_inav->get_altitude()"
AC_WPNav: make member pointer to AP_InertialNav object const since it's
never modified
AC_WPNav: make more member pointers const
AP_InertialNav: move variable definitions to elide unused default
construction of objects (saves 106 bytes)
AP_Buffer: add comments and rename functions to conform with the
STL-container naming conventions
Comment format changes by Randy
AP_InertialNav: rename AP_Buffer functions, fix delay handling bug

The most recent value was used instead of the intended historical value
as indicated by the comment.