Fix issues in extended/behavior/HMagneticField.java

- Include missing `PVector` import
- Fix "naked" `abs()` and `atan2()` (by prepending `Math`). This required a judgment call for the `atan2()` - there's a type conflict for the returns from `getRotation()` (float) and `atan2()` (double). I cast to a `float` based on usage I spotted in HSwarm.java.

Comment: I can't guarantee the accuracy of the `getRotation()` return, based on the fact that I really have no idea what the math is doing here, however I did run through all of the HMagneticField examples to verify that they run and are at least *visually* correct.