DRT-73
- implemented SetAuthorAverageWorkPerDay in AuthorsProcessing.cs;
- added new properties to Summary and Author classes to display average work per day: MonthWorkedPerDay, MonthWorkedPerDayString, SprintWorkedPerDay, SprintWorkedPerDayString;
- save working days for month and sprint in variables and pass them as parameters for methods;
- implemented SetAverageTimeWorkedPerDay, GetTasksCount, SetTotalAverageTimeWorkedPerDay, SetAuthorAverageWorkPerDay;