Add Support for BoatCoach rowing files (CSV)

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from @srwareham in #3280:

If helpful, I've attached a CSV of a Concept2 rowing workout recorded using the BoatCoach Android app. This is the only cloud-free way of capturing second-by-second workout data from an Erg that I've come across and might at least be a good starting point for a data model.

boatcoach_2020-05-06__11-20-26.txt

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Will need a little effort so putting into the 3.7 queue