Integrator double to float and hrt_abstime type useage

[mcsauder]

Describe problem solved by the proposed pull request
The Integrator class integral delta dt is currently created as a double type and then immediately recast in the next line of code to a float type:
const matrix::Vector3f delta_alpha = (val + _last_val) * dt * 0.5f;

This PR simply creates the dt as a float to save resources and employs the hrt_abstime typedef in the obvious instances within integrator.cpp.

Test data / coverage
This PR was test flown on a 250 size quad with pixhawk 4 mini until the battery gave out, tests perform identical to PX4:master branch:

• https://review.px4.io/plot_app?log=b2b3666a-4bb5-4b0d-a5fc-453da051d549

Please let me know if you have any questions on this PR. Thanks!

-Mark

[julianoes]

I think that's fine.

[mcsauder]

Thanks @julianoes!