Avoid including AP_InertialSensor.h in headers

[peterbarker]

increases amount of work compiler needs to do for no good reason

... particularly when including in AP_AHRS.h, which is used a lot.

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
CubeRedPrimary                      8      *           0       0                 -16    0      16
Durandal                            -8     *           -24     -24               -32    -24    0
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     16     *           0       -8                -16    -8     16
MatekF405                           0      *           8       16                0      16     0
Pixhawk1-1M-bdshot                  0                  8       16                -8     8      0
bebop                               32                 16      48                -4     -16    -16
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           0      *           8       8                 0      8      0
skyviper-v2450                                         8                                       

[tpwrules]

Is this a measurable time save? Allowing inline isn't a bad reason. Of course the difference doesn't appear huge, and I am assuming these are not methods on the hot path.

[peterbarker]

Is this a measurable time save? Allowing inline isn't a bad reason. Of course the difference doesn't appear huge, and I am assuming these are not methods on the hot path.

They are also methods of dubious implementation ("first usable" vs "primary").... We're also not particularly good at being consistent about whether the accel vector is a product of the AHRS or ins library.

I have not attempted to measure time deltas. I think it's pretty clear that we won't form needless header dependencies during compilation, on top of any time savings from not including them - and that makes cache hits more likely.

[tpwrules]

Okay, it seems those methods are very rarely used anyway, maybe it would be better to just kill them?

I assume you are testing all the build options, that's the only other possibility of issue I can see.

[andyp1per]

We use inline for a reason

[andyp1per]

Please don't do this, this is in the hot path and inlining helps

[andyp1per]

Simply un-inlining things requires a fair amount of testing, I suggest you don't do that in this PR