Some users might find this an interesting feature. However, I am resistant to merge the current implementation due to two different reasons.

1. It does not make much sense to put the three variables in a vectorfield quantity. This will lead to a non-intuitive visualization of this 'vectorfield' in the GUI or when using mumax3-convert.

2. I do not see the point for having both the in-plane component rxy and the angle theta since rxy=sin theta.

In my opinion it is better to define two scalar fields for the magnetization angles phi and theta, and to not define the in-plane magnetization component rxy.

I agree. I removed the vector field and implemented the theta and phi scalar fields.

@peytondmurray Looks good to me.
Please

• run the unit tests and attach the report;
• please prefix new quantities in engine/angles.go with ext_;
• please rename engine/angles.go to engine/ext_angles.go.

The ext_ thing is a way to highlight contributed functionality.

Okay, I've attached the ext_ prefix to the scalar fields and renamed engine/angles.go to engine/ext_angles.go. Here's the result of running ./test.bash &> test_result.txt.

Cheers!