Add periods of non-wear detection / removal

[rymc]

A nice feature of the library would be a method to detect, and remove, periods of non-wear for wearable devices.

How I have done it in the past with the EurValve wearable is:
"Periods of time where the patient is not wearing the wearable are excluded by measuring the variance in arm angle changes over 20 minute blocks of time. If the variance in a block is less than 1×10−7, then that block of time is excluded from analysis."

Though we may want to try other methods..