Replace heading2D calls with heading

PVector#heading2D has been deprecated, but calls exist throughout a number of PDEs in the codebase.

This issue is for discussing the simple replacement of those deprecated `heading2D` method calls with `heading`.

See issue #1025, #356, and https://github.com/processing/processing-web/issues/13 and processing.core.PVector.java which reads:

```java
  @Deprecated
  public float heading2D() {
    return heading();
  }
```

Note that processing-js has also deprecated this method.