Add to existing trims for RC trim calculation

[dpkoch]

Right now if you've previously set RC trims, but then want to adjust them, you fly with the old trims active and then set a small incremental trim on your transmitter, but then when you call the calibrate_rc_trim service it ignores the old trim and sets only the incremental trim as your new total trim. That's probably not a great behavior.

We could just have users reset the trims to zero before recalibrating, but then you can't do incremental adjustments. So we should change it to having the new trim be added to whatever trim was active while you were just flying.

This is coming from the discussion at https://discuss.rosflight.org/discussion/20/questions-regarding-trimming.

[dpkoch]

At @prasgane, this is the RC trim issue we were talking about. The relevant branch is fix_288

[prasgane]

@dpkoch I just changed the code to fix it but looks like someone already fixed that part. You can close this issue.

[dpkoch]

Hi @prasgane, it's been changed in the fix_288 branch, but not tested or merged in. Did you have a chance to test it with that hex file I sent you?

[prasgane]

@dpkoch tested it this morning and I can confirm that the trim values do not get erased. I am able to add onto the exsisting trim vales.