The shadow method Transformer.transformCollection prevents space.Geometry.Simplify() to work correctly

[andreacomparini]

Using DP geometry simplification I noted that the shadow method tranformCollection() in Tranformer obscure the call at a missing method in DPTransformer and the call at space.Geometry.Simplify() doesn't work in case of geometry collection.
I suppose the following method has to be added for DPTransformer.

func (d *DPTransformer) transformCollection(geom matrix.Collection, parent matrix.Steric) matrix.Steric {
	transGeoms := matrix.Collection{}
	for _, v := range geom {
		transformGeom, _ := d.Transform(v)
		if transformGeom == nil {
			continue
		}
		if d.pruneEmptyGeometry && transformGeom.IsEmpty() {
			continue
		}
		transGeoms = append(transGeoms, transformGeom)
	}
	return transGeoms
}