Add accessors to children arrays from `GeometryArray`

[kylebarron]

Snippet:

pub fn point_child(&self, dim: Dimension) -> Option<&PointArray> {
    let child = &self.points[dim.order()];
    if child.len() > 0 { Some(child) } else { None }
}

pub fn line_string_child(&self, dim: Dimension) -> Option<&LineStringArray> {
    let child = &self.line_strings[dim.order()];
    if child.len() > 0 { Some(child) } else { None }
}

pub fn polygon_child(&self, dim: Dimension) -> Option<&PolygonArray> {
    let child = &self.polygons[dim.order()];
    if child.len() > 0 { Some(child) } else { None }
}

pub fn multi_point_child(&self, dim: Dimension) -> Option<&MultiPointArray> {
    let child = &self.mpoints[dim.order()];
    if child.len() > 0 { Some(child) } else { None }
}

pub fn multi_line_string_child(&self, dim: Dimension) -> Option<&MultiLineStringArray> {
    let child = &self.mline_strings[dim.order()];
    if child.len() > 0 { Some(child) } else { None }
}

pub fn multi_polygon_child(&self, dim: Dimension) -> Option<&MultiPolygonArray> {
    let child = &self.mpolygons[dim.order()];
    if child.len() > 0 { Some(child) } else { None }
}

pub fn geometry_collection_child(&self, dim: Dimension) -> Option<&GeometryCollectionArray> {
    let child = &self.gcs[dim.order()];
    if child.len() > 0 { Some(child) } else { None }
}