Actor message format for Channels

[BenediktBurger]

In pymeasure/pyleco#14 I noticed that we currently have not yet (in PR #56) defined, how to access a channel of a controlled device. (related to #20 , #29 )

I see the following options for the get/set_parameters and call_action methods:

1. They offer a keyword argument "channel" to choose a subchannel. Advantage: It is more simple to implement a check for locked Channels.
2. If a period is encountered in the action/parameter name, the Actor tries to follow that "instance path" (e.g. "channel_1.trace_A.property_X" will be resolved to the property_X of trace_A of channel_1 of the device).
3. We have special methods call_action_channel...

My favorite is option 2, as that allows subchannels etc. seamlessly.