Add get_roll_deg, get_pitch_deg and get_yaw_deg to AHRS interface, start using them

[peterbarker]

Replace with get_roll_rad and get_roll_deg`, both floats.

Now just adds get_roll_deg() etc.; renaming get_roll_rad (etc.) costs over 400 bytes on CubeOrange!

[peterbarker]

I've simplified this PR to simply add the new methods to the interface, not touching the existing creation of the yaw_sensor variables etc.

I'm also not renaming (eg.) get_roll to get_roll_rad in this PR now. This rename costs us 408 bytes on CubeOrange because scripting needs the old binding to exist or we instantly break way too many scripts. We also can't deprecate an alias, it seems. We might still choose to do this in the future (I have the patches :-), but not in this PR.

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                 *
CubeRedPrimary                      -80    *           -168    -184              -32    -88    -64
Durandal                            -40    *           -64     -80               -40    -104   40
Hitec-Airspeed           *                 *
KakuteH7-bdshot                     -48    *           -144    -136              72     -96    -216
MatekF405                           -32    *           -56     -48               -88    -32    -8
Pixhawk1-1M-bdshot                  8                  -40     -40               -64    -24    8
f103-QiotekPeriph        *                 *
f303-Universal           *                 *
iomcu                                                                *
revo-mini                           -32    *           -104    -88               -72    -112   -72
skyviper-journey                                       -104
skyviper-v2450                                         -24

... there's still a bit of low-hanging fruit, too.

[peterbarker]

This should probably be changed to do floating-point division and rounding? Pre-existing problem.

[peterbarker]

.... there's a bug in toy_mode.cpp line 294 which we should fix once this is in.