`Circle` does not require `center` or `radius` properties

[The `Circle` schema](https://github.com/astm-utm/Protocol/blob/cb7cf962d3a0c01b5ab12502f5f54789624977bf/utm.yaml#L297-L304)  currently has optional fields for `center` and `radius`.

The meaning of a circle missing either of these fields is unclear. Can the description of either `Circle` or `outline_circle` be amended to clarify? Otherwise, can these fields be required?