Add functions for reading annotated sleep data

[hofaflo]

The goal is to have functions which download and read all common polysomnography datasets. Each record will be stored in a dataclass containing heartbeat times and annotated sleep stages.

Each reader should be added via a separate PR, this issue gives an overview of the progress.

Datasets:

• [x] MESA (Multi-Ethnic Study of Atherosclerosis) #28
• [x] SHHS (Sleep Heart Health Study) #48
• [x] SLPDB (MIT-BIH Polysomnographic Database) #47
• [ ] MASS (requires registration including project description and ethics approval, so this is not really open)
• [ ] CAPSLPDB (CAP Sleep Database)
• [ ] CinC 2018 (You Snooze You Win - The PhysioNet Computing in Cardiology Challenge 2018)
• [ ] sleep-accel (Motion and heart rate from a wrist-worn wearable and labeled sleep from polysomnography)
• [ ] DREAMS
• [ ] ISRUC-SLEEP Dataset

Planned future extensions:

• metadata
• actigraphy data