prep_data
converts the data frame to a data table and examines the
required columns (RECORDING_SESSION_LABEL, LEFT_INTEREST_AREA_ID,
RIGHT_INTEREST_AREA_ID, LEFT_INTEREST_AREA_LABEL, RIGHT_INTEREST_AREA_LABEL,
TIMESTAMP, and TRIAL_INDEX) and optional columns (SAMPLE_MESSAGE, LEFT_GAZE_X,
LEFT_GAZE_Y, RIGHT_GAZE_X, and RIGHT_GAZE_Y). It renames the subject and item
columns, ensures required/optional columns are of the appropriate data class,
and creates a new column called Event which indexes each unique
series of samples corresponding to the combination of Subject and
TRIAL_INDEX (can be changed), necessary for performing subsequent operations.