Change set_alternate_keys to add_alternate_keys and add error handling

[amontanez24]

Problem Description

There are two desired changes:

1. Update set_alternate_keys to add_alternate_keys because it is possible to add to the existing set.
2. Add error handling to ensure that a primary key cannot be an alternate key

Expected behavior

• Rename set_alternate_keys to add_alternate_keys
• Errors:
  • If the user tries to add a key to the alternate keys when it's already a primary key, raise an error (use one similar to what exists for sequence keys).
  • If the user sets a primary key when it in the alternate keys allow this but raise a warning saying that we will remove it from the alternate keys
    • Be sure to actually remove the keys