add getLatest PM

[Cardy2]

Update SweApi.realtime.context.js - add fix for getLatest PM so that data is automatically associated with a location upon connection

[Cardy2]

opensensorhub#818

[BillBrown341]

The code in #12 looks really good. The only thing I added as minor details is to add comments in the newer methods to help make sense of the stuff for any newer developers. Feel free to disregard my comments since the method names are pretty straight forward.

[Cardy2]

Thanks, I will add some more detailed explanations