Takes a running average of HR data
:param hr: (ndarray) An array of heart rates
:param mins: (int) Number of minutes to take HR over
:returns: An ndarray of average heart rate at each time point
Figures out how many points in mins minutes
:param time: (ndarray) An array of times
:param mins: (int) Number of minutes to take HR over
:returns: The length in data points of mins minutes