Clearing and usercommand of sample settings

ECDC-4143. The user can set and clear the sample fields "temperature", "electric_field" and "magnetic_field" from the command line using "set_sample_fields" and "clear_sample_fields" found in commands/sample.py. The sample fields are also cleared automatically if its current chosen device isn't in the list anymore using "_clear_sample_fields" (gui/panels/devices.py) which is called when the client connects and when the setup changes.