Fixes to allow for custom recording directory outside of ~/.eegnb/

[pellet]

As previously discussed in our meeting, I could not save my recordings outside of ~/.eegnb/
This fixes the issue.
Here is an example of how I set my recordings/data directory using an environment variable.
I modify this environment variable depending on the computer I am running the recording or analysis on, I'm saving my data to a private experiments repository which has a different local directory on each computer.

recording

data_dir = path.join(path.expanduser("~/"), getenv('DATA_DIR'), "data")
save_fn = generate_save_fn(eeg_device.device_name, experiment_name, subject_id, session_nb, data_dir)

loading

data_dir = path.join(path.expanduser("~/"), getenv('DATA_DIR'), "data")
raw = load_data(subject,session,
                experiment='visual_n170', site='local', device_name='synthetic',
                data_dir = data_dir)

[JohnGriffiths]

@pellet good to convert this to a full PR