Add the EEGLAB and EEGLAB-HJ coordinate systems

[sappelhoff]

came up on discourse

EEGLAB and EEGLAB-HJ are supported in BIDS, and EEGLAB-BIDS will soon write data this way.

Should be done here: https://github.com/mne-tools/mne-bids/blob/4c0755c6eb567c7840d70acf06f22201c5e75c65/mne_bids/config.py#L196-L197

... perhaps with a little utility function that tries to convert theses systems to head.