deprecate public getters/setters for properties of `GEOSGeometry` and its subclasses

[Claude Paroz]

I would let num_points and crs, they make no harm.

I see get_srid, set_srid as an artifact from the time the property decorator didn't exist. So +1 to use the modern decorator syntax and deprecate the get/set methods.

[Sergey Fedoseev]

I was motivated to deprecate num_points and crs because number of GEOSGeometry members looks disturbing for me.

In [2]: GEOSGeometry.
In [2]: from django.contrib.gis.geos import GEOSGeometry as Geometry

In [3]: Geometry.
Geometry.append            Geometry.geojson           Geometry.prepared
Geometry.area              Geometry.geom_type         Geometry.ptr
Geometry.boundary          Geometry.geom_typeid       Geometry.ptr_type
Geometry.buffer            Geometry.get_srid          Geometry.relate
Geometry.centroid          Geometry.has_cs            Geometry.relate_pattern
Geometry.clone             Geometry.hasz              Geometry.remove
Geometry.contains          Geometry.hex               Geometry.reverse
Geometry.convex_hull       Geometry.hexewkb           Geometry.ring
Geometry.coord_seq         Geometry.index             Geometry.set_srid
Geometry.count             Geometry.insert            Geometry.simple
Geometry.crosses           Geometry.intersection      Geometry.simplify
Geometry.crs               Geometry.intersects        Geometry.sort
Geometry.difference        Geometry.json              Geometry.srid
Geometry.dims              Geometry.kml               Geometry.srs
Geometry.disjoint          Geometry.length            Geometry.sym_difference
Geometry.distance          Geometry.mro               Geometry.touches
Geometry.empty             Geometry.normalize         Geometry.transform
Geometry.envelope          Geometry.num_coords        Geometry.union
Geometry.equals            Geometry.num_geom          Geometry.valid
Geometry.equals_exact      Geometry.num_points        Geometry.valid_reason
Geometry.ewkb              Geometry.ogr               Geometry.within
Geometry.ewkt              Geometry.overlaps          Geometry.wkb
Geometry.extend            Geometry.point_on_surface  Geometry.wkt
Geometry.extent            Geometry.pop 

I'll change ticket summary because there are also getters/setters for coordinates of Point and probably there are others.