Support day of year initialization and formatting

[ChristopherRabotin]

ISO8601 allows epochs to be initialized from the day in the year. This should also be supported in Hifitime. The initialization should be trivial to implement, but the formatting may be a notch more complicated.

The formatting should be performed via its own method, independently of fmt::Display or one of the standard tokens since all of these are used already.

Hence, it's likely that a new formatting struct be needed to store the desired format.

• Add from_day_of_year and year_days_of_year
• Build custom formatting structure