[FIX] Fix bug in electrode channel extraction
[FIX] Fix bug in electrode channel extraction

Prior to this commit, the `extract_connections` function would behave
unexpectedly when multiple electrodes mapped to the same channel(s).

 - Scenario:
     * Multiple electrodes in SVG device file mapped to the same
       channel(s) (i.e., have same value set for `"data-channels"`
       SVG element attribute).
 - Expected behaviour:
     * `df_electrode_channels` contains entries for all electrode
       channel mappings.
 - Observed behaviour:
     * `df_electrode_channels` only contains entries for a single
       electrode out of a set of electrodes mapped to the same
       channel(s).

As of this commit, the expected behaviour occurs.

wheeler-microfluidics/microdrop