Add reader for SHHS

[hofaflo]

SHHS

This is pretty similar to read_mesa, except for the following differences:

• SHHS is grouped into two "visits" (there are two recorded nights for some participants), therefore there is an additional directory level (shhs1/shhs2).
• For MESA the ECG channel name is "EKG", for SHHS it's "ECG".
• SHHS does not include actigraphy data (while read_mesa doesn't read that data yet, this is planned).

Due to the similarity, large parts of read_mesa and read_shhs could be put into a new function. I'd like to postpone refactoring this until read_mesa handles actigraphy data and both functions handle metadata.

[cbrnr]

Thanks @hofaflo!