Error subsetting data when only 1 acceleration metric calculated

[jhmigueles]

Bug identified when investigating this question from the Google group: https://groups.google.com/u/1/g/RpackageGGIR/c/u0YSqdv3AuU

When reading actiwatch_csv data, only 1 acceleration metric is added to part 1 milestone data. In part 2, g.analyse.perday subsets the time series into day chunks, but line 286 in that function tries to get the length of vari using nrow. Since vari is coerced to numeric (because there is only one acceleration metric in the data frame), then nrow function doesn't work.