conflicting names between tsv and json

[robertoostenveld]

@dorahermes mentioned in an email that "Currently, for iEEG only electrodes is an OrderedDict(), but we have an _electrodes.tsv and _electrodes.json. Should we change the name for _electrodes.json? Since any .tsv can have an accompanied .json that specifies the column indices, I think this may be necessary..."

General BIDS specifies that for any xxx.tsv there can be an accompanying file xxx.json with the data dictionary (i.e. description of the columns).

This potential naming conflict should be checked for MEG, EEG and iEEG.