Implement annotations

[philipmjohnson]

Please do work for this task in a branch called issue-99.

To make power quality events actionable, it can be important to understand the relationship between them and other things happening in the environment. Annotations are a building block for creating these relationships.

The concept of an annotation is simple: it specifies some sort of external information that might be relevant to power quality during a certain time interval at a certain location. So, for a collection named "annotation", the fields would be:

• start_timestamp_ms: start time for the behavior represented by this annotation.
• end_timestamp_ms: end time for the behavior. If not present, then the behavior is ongoing.
• location: Either location or region must be specified, but not both. Location is a string (slug).
• region: Either location or region must be specified, but not both. Region is a string (slug).
• type: A string indicating the type of annotation.
• description: A string providing details about the behavior.

Annotations can be supplied by people (there was a thunderstorm today during these anomalies), or other tools (an insolation system could provide annotations of type "insolation" to provide solar data.

The above fields can be extended by additional fields as necessary for any specific type of annotation.

[philipmjohnson]

This is now part of the implementation of Incidents and might be much more complicated and/or replaced by some sort of "context" object, so I'm closing this issue with the expectation that we'll create a different one when we get to an appropriate point in development.