Avoid warnings about type-punned

[markub3327]

Hello,

you used for estimating inverse square root old solution and modern compiler gives me warning:

/Users/martin/Documents/Arduino/libraries/Adafruit_AHRS/src/Adafruit_AHRS_Mahony.cpp: In static member function 'static float Adafruit_Mahony::invSqrt(float)':
/Users/martin/Documents/Arduino/libraries/Adafruit_AHRS/src/Adafruit_AHRS_Mahony.cpp:265:22: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   long i = *(long *)&y;
                      ^
/Users/martin/Documents/Arduino/libraries/Adafruit_AHRS/src/Adafruit_AHRS_Mahony.cpp:267:18: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   y = *(float *)&i;
                  ^
/Users/martin/Documents/Arduino/libraries/Adafruit_AHRS/src/Adafruit_AHRS_Madgwick.cpp: In static member function 'static float Adafruit_Madgwick::invSqrt(float)':
/Users/martin/Documents/Arduino/libraries/Adafruit_AHRS/src/Adafruit_AHRS_Madgwick.cpp:286:22: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   long i = *(long *)&y;
                      ^
/Users/martin/Documents/Arduino/libraries/Adafruit_AHRS/src/Adafruit_AHRS_Madgwick.cpp:288:18: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   y = *(float *)&i;
                  ^

The better way is using union as a trick. Thanks.

[tyeth]

Thanks for your contribution @markub3327 ❤️
It's been effectively absorbed in #36 and released as 2.3.5