Define interface `Comparable` method `compareTo` as "canonical order" in more `FieldElement` derived classes

[axkr]

A Comparable interface should be established for a "canonical order" for more FieldElement derived classes.

The Binary64method already has a Comparable interface:

• https://github.com/Hipparchus-Math/hipparchus/blob/4829f0e2d4997a5790a8477c546b7c41c7e069bf/hipparchus-core/src/main/java/org/hipparchus/util/Binary64.java#L256

For the Complexclass I would like to suggest:

public int compareTo(final Complex o) {
    int c = Double.compare(this.getReal(), o.getReal());
    if (c != 0) {
      return c;
    }
    return Double.compare(this.getImaginary(), o.getImaginary());
}