Move sample annotations into separate table

[jchester]

As part of a moral crusade against nulls in databases.

• Do the needful
• Update readme example

[jchester]

Open design questions:

1. How do we annotate a range of samples? Do we want to?
2. Are overlapping annotations on samples a problem?
3. Should each sample be limited to one annotation, or can each sample have many?
4. Should include_in_limit_calculation also be moved out into another table?
5. Are annotations on samples or between samples? Or do we need to support both?