Add sample datasets

[idfah]

Description

In order to get moving with CEBL quickly, it would be nice to have some small, sample datasets included in the repo.

Acceptance Criteria

• sample p300 data included in repo
• sample mental task data included in repo
• simple functions in eeg to load sample EEG data

Tasks

• inquire about using CSU datasets
• upload datasets
• create loading functions in eeg submodule