include paramter for which ecube headstage to load

also note that currently all the channels will be loaded from all the headstages. this may not be the desired behavior in the future when we're running experiments with multiple headstages or multiple experiments simultaneously. need to also rethink the way load_ecube_metadata() and load_ecube_data() work so that they include information about which headstage to load

_Originally posted by @leoscholl in https://github.com/aolabNeuro/analyze/issues/60#issuecomment-910557067_