How to handle the fes2:SpatialDescription element

The fes2 standard defines the Union type SpatialDescription, allowing for objects of type:

• geometry(GM_Object)
• envelope(GM_Envelope)
• ValueReference

It also states that literal geometric values shall be expressed using GML.

In order to keep pyfes' implementation simple, it would be preferable to not have to implement custom Geometry classes. Presumably the clients of pyfes will be doing this already.

As such pyfes implements a GML to WKT parsing mechanism. This means that pyfes will be able to parse GML input into WKT (perhaps EWKT is more useful).