Add API for querying supported input coupling modes

[azonenberg]

If the user requests 50-ohm input on a scope that doesn't have it, most drivers will no-op and maybe print a message to the console.

Ideally, there should be a way for drivers to communicate that the (low end) scope has high-impedance inputs only, and disable/hide the 50 ohm option in the UI.

[azonenberg]

Some scopes don't have ground coupling either. We should have a new API in the Oscilloscope class that returns a set so UIs can hide unsupported options.

[azonenberg]

Proposed interface: virtual std::setOscilloscopeChannel::CouplingType GetAllowedChannelCoupling() =0;

[azonenberg]

Also related: Siglent scopes (at least SDS2000X+) support AC coupled 50 ohm inputs!