Columns with iterables given directly in the catalog spec

[aulemahal]

Change Summary

Adds a "iterable_columns" field in the ESM Catalog Spec that acts the same as the columns_with_iterables argument of the esm_datastore.

The ESMCatalogModel object has a columns_with_iterables property that returns the actual list of columns containing iterables, which could be a superset of iterable_columns if converters were given in the read_kwargs. Thus, I had to choose a different name for the config-provided version.

Related issue number

Fixes #595

Checklist

• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable (one how-to page can be updated)

I gathered that deprecating columns_with_iterables would be much too breaking. This means the same thing can be done in two ways. Is this ok?

I only added a small test. The test suite already makes a good job testing the "iterable" feature, I only tested the catalog opening when the new option is given.