Rename "imp" column to "impedance" in ElectrodesTable

[rly]

This minor QOL issue came up in a data conversion project. "impedance" is much more readable. We could change that only in the API, but I think it makes sense to change it in the schema. Deprecate but keep the "imp" column, add an "impedance" column in the schema. Have PyNWB warn when users write the "imp" column.