Use macros for more convenient component building

[wsphillips]

There should be an easier and more readable way to specify gating variables, channels, compartments, etc.

Ref https://github.com/wsphillips/Conductor.jl/pull/8#issuecomment-907722134 for beginnings of this.

[wsphillips]

These should be revised for compatibility with the new ConductanceSystem/CompartmentSystem and potential extended for network construction (i.e. NetworkSystem)