Utilities for merging tables

[smmaurer]

🎉 ISSUE 100 🎉

This is to sketch out functionality for validating implicit merge relationships, and performing merges. See issues #78 and #94.

utils.validate_table()

Check some basic expectations about the table:

• unique index or multi-index
• if columns match names of other indexes, do they make sense as join keys?
• validate against an orca_test spec if available

This functionality is already implemented in the LoadTable() template's validate() method -- we can just pull it out.

utils.validate_all_tables()

Run for all tables.

I think the validation will not be performed automatically when tables are registered, because of performance, but we can recommend that users run the validation at some point that makes sense for their workflow.

utils.merge_tables()

Replaces orca.merge_tables(), using implicit join keys instead of "broadcasts". Should support multi-indexes.

Stricter and more deterministic than Orca:

• tables merged in listed order
• requested column names must be unique