Item accessor operator and length in geometry and feature collections

Some new helper methods.

GeometryCollection:

  /// The number of geometries in this geometry collection.
  ///
  /// See also [geometries].
  int get length;

  /// The geometry at the given [index] in this geometry collection.
  ///
  /// The given [index] must be non-negative and less than [length].
  ///
  /// See also [geometries].
  E operator [](int index);

FeatureCollection:

  /// The number of features in this feature collection.
  ///
  /// See also [features].
  int get length;

  /// The feature at the given [index] in this feature collection.
  ///
  /// The given [index] must be non-negative and less than [length].
  ///
  /// See also [features].
  E operator [](int index);

navibyte / geospatial

Item accessor operator and length in geometry and feature collections #230