move FieldVariable.get_data_shape() functionality to Field subclasses

- new FEField.get_data_shape(), IGField.get_data_shape()
remove Approximation.get_v_data_shape(), .get_s_data_shape()

- update .describe_geometry()
fix 'surface_extra' assembling cells in Term.get_assembling_cells()

- for Field.get_econn() in FieldVariable.evaluate()