ENUM conventions: Allow exceptions for "technical enums"

[RIFJo]

The enum naming conventions have rules for required fields.

"The first enum field shall always use the suffix UNKNOWN. The second enum field shall always use the suffix OTHER."

This requirement, especially the presence of the "UNKNOWN" field is not useful for some enums with technical context - there are cases where it is correct to have a clearly defined default value, using the suffix "DEFAULT = 0" as first entry.

discussions here and here

We (the sensor modelling discussion group) suggest to rewrite the requirements to allow exceptions - for example

• when a widely accepted default value exists
• having "UNKNOWN" is harmful
• the enum is "complete" and no value for "OTHER" is going to be needed

[pmai]

Closed by #72.