Building fall-backs so 'task_events' defaults to 'events' if 'task_events' can't be found (and vice versa).

[Riley16]

cmlreaders/cmlreaders/readers/readers.py

Line 161 in c4b1935

"all_events", "events", "math_events", "ps4_events", "task_events",

There are strange edge cases where for some experiments (e.g., OPS), 'task_events' or 'events' will not load any events, while the other form of events will load events. We should make this robust so that if one is missing, the other (or 'all_events') is checked and loaded with a warning.