ENH: Add enum support

[abel-bzz]

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs (you can delete this text from the final description, this is just a guideline):

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

[summarize your pull request here]

Changes

• Make it possible to parse EnumTypeDef and turn the enums into CF flag_values and flag_meanings.
• Also add support for scalar variable (when shape is empty)

References

I. For Enum type conversion: from common_data_model_overview

There are 3 enum types: enum1, enum2, and enum4, corresponding to storing the integer as a byte, short, or int.

II. For scalar variable: from annotated_ncml_schema

The shape attribute lists the names of the dimensions the variable depends on. For a scalar variable, the list will be empty.

[huard]

All good, could you add a note in the CHANGELOG, under a new 0.4 release ?

[abel-bzz]

All good, could you add a note in the CHANGELOG, under a new 0.4 release ?

Oops I did it and forgot to push the commit