Support Enums

[Stiivi]

Add support for enumerations.

Enum type: category, name, label, description, type(?)

Enum entry: name, value/tag, label, description

To do:

• Enumeration meta-model structures
• Type support
• CSV reader: enums.csv
• Python writer

Note for the Python writer: in the __init__ method we need to consider special initialization before the python/mypy#741 is closed.