Clarify Policy priority requirements

[zanebclark]

The requirements around Policy priority came up during the 2/22 meeting.

Two Policies that apply to the same Curb Zone with overlapping Time Spans and user classes MUST NOT have the same priority.

@schnuerle and @jacobmalleau suggested that two policies w/ overlapping time_spans and overlapping user_classes could have the same priority so long as the user_classes arrays don't consist of the same values. (I was going to say, "so long as the user_classes arrays aren't equivalent". See #82 for more on this). If this is the intent of the spec, we could reword the above requirement:

Two Policies that apply to the same Curb Zone with overlapping Time Spans and equivalent User Class enumerations MUST NOT have the same priority.

[jacobmalleau]

I agree with this.

[schnuerle]

I've updated the priority definition with your clarified language, thanks!

https://github.com/openmobilityfoundation/curb-data-specification/blob/release-1.0.0/curbs/README.md#policy