Checks the ratelimit values after the last request and stores the left requests in a global variable.
ratelimit(req)value returned from the GET function, used internally in other functions
A global variable usage_left shows the current limits.
Requests to the Strava API are rate-limited. The default rate limit allows 600 requests every 15 minutes, with up to 30,000 requests per day. See the documentation at https://strava.github.io/api/#access.