SCPIInstrument API to list driver supported models and transports

[fredzo]

Hi Andrew,

This PR is an extension of SCPIInstrument API used to provide information about models supported by the driver and accepted transports.
AddInstrumentDialog (PR to come once ngscopeclient/scopehal-apps#951 has been merged) relies on this API to pre-fill transport combo, path string and nickname:

Driver may add a static GetDriverSupportedModels() to list supported model names and their associated transport / connection string template.
If they do, this list will be used in AddInstrumentDialog to limit the list of available transports and to pre-fill nickname and connection string.
If they don't, the behavior of the AddInstrumentDialog remains unchanged.
If a driver lists more than one model, a model combo is added to the dialog.

static std::vector<SCPIInstrumentModel> GetDriverSupportedModels()
{
	return {
	{"Owon XDG 2000", {
		{ SCPITransportType::TRANSPORT_LAN, "<ip_address>:3000" },
		{ SCPITransportType::TRANSPORT_USBTMC, "/dev/usbtmc<x>" },
	}},
	{"Owon XDG 3000", {
		{ SCPITransportType::TRANSPORT_LAN, "<ip_address>:3000" },
		{ SCPITransportType::TRANSPORT_USBTMC, "/dev/usbtmc<x>" },
	}}
	};
}

This PR also fixes:

• Ridden RD and DPS model support ( Riden rd6024 current setpoint is off by a factor of 10 #1013 )
• Attenuation value reading for Magnova driver