Reintroduce physical channels

[alecandido]

In v0.2 the physical channel has been completely replaced by the logical one, which is most often the object actually used.

Still, there are residual applications for which the physical channel is the object actually involved, such as specifying the connections, and some of its properties. Ignoring them, leads to duplication of the same information across the different logical channels multiplexed on the same physical link.

Examples

Gain/attenuation may be an example of duplicated info (e.g. when multiple resonators are multiplexed on one line, or there are multiple transitions calibrated on a drive).

While even the DC offset would be naturally a property of a physical line, it would not be duplicated because DC channels do not host multiple logical ones.