Define more specific return type on newWith and newFrom methods of Point sub classes

[navispatial]

The abstract Point class following factory methods to create new point instances of the same sub type methods are called on.

  Point newWith({num x = 0.0, num y = 0.0, num? z, num? m});
  Point newFrom(Iterable<num> coords, {int? offset, int? length});

Sub classes like Point2 implement these methods like:

  @override
  Point newWith({num x = 0.0, num y = 0.0, num? z, num? m}) =>
      Point2(x: x, y: y);

  @override
  Point newFrom(Iterable<num> coords, {int? offset, int? length}) {
    CoordinateFactory.checkCoords(2, coords, offset: offset, length: length);
    return Point2.from(coords, offset: offset);
  }

This should work fine.

However if you are using a specific Point sub type, like Point2, then it might be useful to define more specific return type.

That is, for Point2 the implementation would change to following:

  @override
  Point2 newWith({num x = 0.0, num y = 0.0, num? z, num? m}) =>
      Point2(x: x, y: y);

  @override
  Point2 newFrom(Iterable<num> coords, {int? offset, int? length}) {
    CoordinateFactory.checkCoords(2, coords, offset: offset, length: length);
    return Point2.from(coords, offset: offset);
  }

[navispatial]

Implemented on the pre-release 0.8.0-a.2 of the upcoming 0.8.0 release