Boids io_altitude_idx_ parameter set incorrectly

[crebhuhn]

Objective: Continue to use Boids autonomy with no neighbors (goal seeking only)

Problem: in Boids.cpp, line 261:
vars_.output(io_altitude_idx_, v(2));

This sets 'altitude' to a value that is indexed to speed. This will cause a diving behavior if present altitude is >1. This is unexpected behavior and inconsistent with the other control modalities. Three possible fixes:

1. remove the option to provide Boids control using desired altitude (it's velocity-based originally I believe)
2. set the desired altitude by position + some scaled velocity:
   vars_.output(io_altitude_idx_, state_->pos().z() + v(2)*zscale);
3. set the altitude based on the goal position
   vars_.output(io_altitude_idx_, goal_(2));

Commit: 3b1b119

Recreated by using Boids autonomy with heading/speed/alt controller.

[SyllogismRXS]

I think option 2 is probably the most intuitive. I tested something similar and pushed it the boids-alt branch. (https://github.com/gtri/scrimmage/tree/boids-alt). Try out that branch let me know if it works for you. I had to update the predator_prey_boids.xml mission file as well.