Add accuracy field to GPS telemetry

[marie-x]

Explain pull request

Per #260

Some software configurations don't provide an HDOP figure, but do provide an accuracy measurement in meters. For example, both iOS and Android platforms do this. At least one operator has Android running on the vehicle, and at least one operator has used the user's smartphone to collect some GPS data.

Rather than asking operators to back out HDOP in these situations, the spec should support receiving a "radius of accuracy in meters."

Is this a breaking change

A breaking change would require consumers or implementors of the API to modify their code for it to continue to function (ex: renaming of a required field or the change in data type of an existing field). A non-breaking change would allow existing code to continue to function (ex: addition of an optional field or the creation of a new optional endpoint).

[ ] Yes, breaking
[x] No, not breaking
[ ] I'm not sure

Provider or agency

Which API(s) will this pull request impact:

[ ] provider
[x] agency
[ ] both

Additional context

Add any other context or screenshots about the feature request here.

[hunterowens]

this needs associated JSON schema update?

[hunterowens]

^ bump