Add new Fitbit activity fragmentation and walking cadence features

[jenniferfedor]

This PR adds to the Fitbit steps intraday RAPIDS provider:

• Activity fragmentation features based on papers by Di et al., 2017, Schrack et al., 2019, and Wanigatunga et al., 2019: active-to-sedentary transition probability; total duration of active bouts less than 5 minutes, between 5 and 10 minutes, and greater than 10 mintues; Gini coefficient
• Walking cadence features based on a review by Tudor-Locke et al., 2018: mean and uncensored mean walking cadence; peak 1-, 30-, and 60-minute cadence; maximum 5-, 20-, 30-, and 60-minute cadence; total minutes in various cadence bands
• A parameter that allows the user to define the number of consecutive minutes with zero step counts above which a period of time will be labeled as device non-wear time. This is used to compute the uncensored mean cadence feature
• A parameter that allows the user to drop from feature computation rows of raw data with minute-level step counts above a specified threshold (e.g., some physically implausible value such as 1000 steps/minute).
• Updates to tests for Fitbit steps intraday features
• Updates to documentation for Fitbit steps intraday features