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

This revealed a possible error in AP_InertialNav.
AP_InertialNav: rename AP_Buffer functions, fix possible bug:

line 334 & 335:

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