Request: make simplekml.coordinates.Coordinates indexable

[IWinarto]

This request is about PointGeometry, which is the parent of a few featgeom classes.

Currently, to modify PointGeometry.coords, one has to assign a new iterable of coordinates to that property as follows: PointGeometry.coords = new_coordinates. However, this instead creates a new simplekml.coordinates.Coordinates object that contains all the coordinates in new_coordinates, but simplekml.coordinates.Coordinates does not support indexing.

Thus, if the user wants to modify a portion of the coordinates, the user would need to create another simplekml.coordinates.Coordinates object again by assigning PointGeometry.coords = another_new_coordinates, which is not as straightforward and as clear (or readable) to do as simply indexing it: PointGeometry.coords[x:y] = some_coordinates.

It would be much more useful if Coordinates can be modified via indexing (to modify the hidden _coords attribute of the class).