add event feature

Add a new `event` feature. This would be implemented similar to `session` variable and would serve as a tag/label for tagging event onsets, behavioral and experimental events . Also add it as a loadable subfield.

Start by locating how the `session` variable is implemented and how loadable subfields are defined. Add the analogous `event` feature for tagging event onsets, behavioral events, and experimental events, and verify that it can be loaded as a subfield.

Written by the indexing model from the issue text.