Add customizable check digit handling

[gredler]

We may want to add the option to customize how we handle check digits in symbols which use them:

• Prohibit user-provided check digit, system always calculates it (this is the current behavior, I think)
• Require user-provided check digit, system throws exception if incorrect
• Require user-provided check digit, system silently corrects it if incorrect
• Allow (but not require) user-provided check digit, system throws exception if provided and incorrect
• Allow (but not require) user-provided check digit, system silently corrects it if provided and incorrect

We need to think through the implications, though, and consider whether the additional complexity is worth it...