transparent `ShapesModel.parse` geometry arguments

[MobiTobi]

The geometry argument is used to select which kind of shapes to create when using ShapesModel.parse. The possible arguments are 0, 3 and 6 which map to Circles, Polygon and MultiPolygon respectively.

Instead of relying on an incomplete enumeration of GEOS geometry types the argument should accept strings ( "circles", "polygon", "multipolygon").

[LucaMarconato]

@giovp I also like this proposal, could you implement it please when doing the io refactoring?