[ENH] BEP042 - EMG examples

[neuromechanist]

The emg-example dataset contains the examples discussed during the biweekly meetings of the BEP042 team @bids-standard/bep042. Each subject is one of the examples from the metadata document.

Here are some key metadata discussed which might be different from other modalities:

1. channels.tsv optionally includes placement and group columns borrowed from Motion and iEEG, respectively.
2. channels.tsv also optionally includes signal_electrode column, indicating which electrode(s) was used (in conjunction with the reference) to make the channel.
3. Multiple coordinate systems can be added to the channels.tsv and electrodes.tsv to adequately describe the channel or electrode placement.
4. The coordinate systems can be optionally linked together using Anchor electrodes.
5. A loc_reference column is optionally added to indicate which coordinate system is used for [x,y,z] columns in electrodes.tsv or the placement column in channels.tsv.

We appreciate your input and feedback on the examples. You can comment about specific examples here, on the original issue bids-standard/bids-specification#1371, or if needed the PR bids-standard/bids-specification#1998