Add events support

[brendanjohnharris]

This PR does two key things:

1. Adds an events field to the File type to support loading _events.tsv files as DataFrames
2. Updates compatability requirements for newer Julia versions

Docs and tests have also been added where relevant, and helper functions like sessions to access fields without using . syntax.

Non-whitespace changes

[darrencl]

Thanks a lot for submitting this PR! This is definitely useful and I am happy to know that someone actually use this tool! 😄

I am guilty for not checking this sooner as I have not been actively using Julia myself recently – I apologise for that.

Overall, this looks good to me, but I just have 1 question & possible change request in the comment.

[darrencl]

If I understand correctly, get_events_path(path) could return nothing if for some reason the corresponding _events.tsv doesn't exist.

If that's the case, the CSV.read(nothing) will raise MethodError: no method matching joinpath(::Nothing), which could be unhelpful for user.

I suggest raising a more useful error in get_events_path(path) that indicates joinpath(this_dirname, fname_without_ext * "_events.tsv") couldn't be found.