Simplify the Type method of the Geometry type

No need for the switch statement since we already have direct access to
the geometry type. The switch statement is likely a remnant of
extracting out geometry type (a _long_ time ago). The only functional
difference here is that if a Geometry is somehow constructed with an
invalid geometry type (which should be impossible), then the `Type`
method will no longer panic but instead just return the invalid geometry
type.

geom/type_geometry.go

@@ -54,26 +54,9 @@ func (t GeometryType) String() string {
 	}
 }
 
-// Type returns a string representation of the geometry's type.
+// Type returns the type of the Geometry.
 func (g Geometry) Type() GeometryType {
-	switch g.gtype {
-	case TypeGeometryCollection:
-		return g.MustAsGeometryCollection().Type()
-	case TypePoint:
-		return g.MustAsPoint().Type()
-	case TypeLineString:
-		return g.MustAsLineString().Type()
-	case TypePolygon:
-		return g.MustAsPolygon().Type()
-	case TypeMultiPoint:
-		return g.MustAsMultiPoint().Type()
-	case TypeMultiLineString:
-		return g.MustAsMultiLineString().Type()
-	case TypeMultiPolygon:
-		return g.MustAsMultiPolygon().Type()
-	default:
-		panic("unknown geometry: " + g.gtype.String())
-	}
+	return g.gtype
 }
 
 // IsGeometryCollection return true iff the Geometry is a GeometryCollection geometry.