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:

• hipparchus/hipparchus-core/src/main/java/org/hipparchus/util/Binary64.java

  Line 256 in 4829f0e

  public int compareTo(final Binary64 o) {

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());
}