Slicing in SurprisalArray is not fully Pythonic

[aalok-sathe]

Need to either: [1] make a note somewhere or [2] add a warning or [3] add a workaround implementation that slicing doesn't exactly work the same way as it does with Python lists or numpy arrays.

• [0:None] has undefined behavior
• [:] has undefined behavior
• [x:-1] has undefined behavior

What does work: providing actual or overshooting indices to characters or words within the stimulus/input.

• [1:3, 'char'] works fine and returns surprisal over all tokens overlapping with chars 1:3
• [0:99, 'char'] works fine and returns surprisal over all tokens that appear within the first 99 chars