#27, #33 - Calculate current average speed by summing the straight-line
distance between each of the last five track points and dividing by the
elapsed time between the first and last points in the subset.  While
this version runs in Verifier, I still need to:
- determine whether the calculated speed is correct
- fix pace (currently displays as 0)
- test boundary cases (0, 1, 2, and 3 track points)
As part of this work I eliminated the Location structured data type, as
leaving it in place will teach the students bad habits.