parsing channels/channel references into purpose-built data structures rather than symbols/strings

[jrevels]

It'd be ideal to have something like a string macro e.g. ch"left-eeg.m1" that nicely parses things for you, so that we could conveniently pretty-print the value as the string macro

[jrevels]

I'm semi-convinced we should just use something like https://github.com/JuliaSymbolics/SymbolicUtils.jl for this; channel names would just be elements from that package and then we'd overload very basic arithmetic/accessor operations (+, -, mean, ., etc.) between Samples to apply the correct transformation to the channel names. We could then loosen the restrictions in the Onda format to allow for things like + and mean.

[jrevels]

I think this is obsoleted as of #69