Switch from day of year index to month-day index across package

This MR adjusts percentile calculations away from using day of year to using month-day. Using a month-day index makes it more transparent how and when leap days are used in the percentile calculations. This MR does not get rid of the creation of a day of year index column, so it is still available to users, but it isn't leveraged by the hyswap package or plots.

This is a reimagination of !56 (closed), which at one point had a buggy issue that caused me to use @shamshaw's sdh-doy-handling branch to continue edits.

This MR should address the following issues:

#70 (closed) , #71 (closed) , #61 (closed) , #4 (closed) , #6 (closed)