New TaxonomySpec class to pass to create_taxonomy().

[licleary]

To create resources we use specs, for example, the method

tamr_unify_client.categorization.category.collection.CategoryCollection.create()

takes as input a CategorySpec class to create a new category in a taxonomy.

To create a taxonomy in a categorization project we have the method

tamr_unify_client.categorization.project.CategorizationProject.create_taxonomy()

However, no TaxonomySpec class exists.

To be consistent, it would be nice to have a TaxonomySpec class.