fix: choice of the value of the notification state

[cmotelet]

The purpose of this PR is to clarify the possible values for the states in notifications vs the states in zones.

• For a notification, the possible states are: [normal|alert|warn|alarm|emergency] (deletion of nominal).
• For a zone, the possible states are: [nominal|normal|alert|warn|alarm|emergency] (no change).

The file gitbook-docs\data_model_metadata.md#alarm-management was already updated in the documentation.

Please comment to find the best compromise.

[tkurki]

Hmm. Zones and notifications are connected in the spec: https://signalk.org/specification/1.3.0/doc/notifications.html#expected-implementation-behaviour

If we have zones that define normal and nominal zones they are treated like the more serious notifications: when the value is in that zone the notification is set. I don't think we have anywhere specified that normal and nominal are special cases and should not set notifications values.

So I don't think anything needs to be changed in the spec.

[cmotelet]

cancelled following tkurki's explanations