`python`: Improve API for discrete grid property

[alifbe]

Improve API related to discrete grid property:

• Identify if a grid property is a discrete or continuous
• If a grid is discrete, provide API to get discrete_property_category_names (reverse of https://api.resinsight.org/en/next-major-release/PythonExamples_case_and_grid_operations.html#set-discrete-active-cell-property