rename ins get_primary_accel to get_default_accel

[peterbarker]

We agreed on DevCall to rename to get_first_usable_accel.

get_default_accel might be better in that a user could have a parameter specifying their primary accelerometer to be used

I have a branch which renames to get_first_usable_accel, too...

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
Durandal                            *      *           *       *                 *      *      *
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     *      *           *       *                 *      *      *
MatekF405                           *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot                  *                  *       *                 *      *      *
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           *      *           *       *                 *      *      *
skyviper-v2450                                         *                                       

[andyp1per]

I want get_first_usable_accel() I think its clearer