Implement broadcastable for Samples

[omus]

By implementing broadcastable(::Samples) we can avoid having to use Ref when using Samples during a broadcast operation (e.g. channel.(Ref(eeg), rows)).

[ararslan]

Don't we only avoid needing to use Ref if we define Samples to broadcast as a scalar? I think I would find it surprising as a user for it to broadcast as a scalar; I think I'd expect it'd broadcast like the type of its data field.

[jrevels]

this is borderline superseded by #77