Add signal_type attribute to ElectricalSeries.data

[stephprince]

cc @pauladkisson

See the original discussion here: NeurodataWithoutBorders/nwbinspector#579.

The proposal is to add 'signal_type' as an attribute to ElectricalSeries.data that would be enumerated text ("eeg", "ecg", "emg", "lfp", etc.) to distinguish between different types of bioelectric signals stored as ElectricalSeries.

[h-mayorquin]

I think it would be better to have dedicated types for these categories as they are so prominent in experiments and might bifurcate later. Can't see any strong downside of having an enum type in the meanwhile but I also can't see the use of it as that can be easily conveyed on the name or description.