Add demo and integration test with istSOS

[nuest]

No description provided.

[nuest]

@massimiliano-cannata @mantonovic is there a list of publicly available istSOS instances? I would like to create a little demo and make sure our R package works well with instances of your service implementation, both with a few integration tests and a connection/listing in https://github.com/nuest/sos4R/blob/feature/0.4/vignettes/sos4R-vignette-99-services.Rmd

[massimiliano-cannata]

@nuest there's no list of public services, but you can use the demo, which includes some real stations (both location and data) for istSOS2 compliance testing. The service is available at: [http://istsos.org/istsos/demo] (http://istsos.org/istsos/demo) while here http://istsos.org/istsos/demo you have the associated administrative graphical interface.