toNepaliMonthCalendar
Converts a CustomCalendar object to a NepaliMonthCalendar object.
This function extracts the year, month, total days in the month, first day of the month, and last day of the month from the CustomCalendar and creates a new NepaliMonthCalendar object with those values.
Return
A NepaliMonthCalendar object representing the same month as the CustomCalendar.
Narrows this month to the Bikram Sambat-only NepaliMonthCalendar.
The year and month are copied verbatim, so calling this on a CalendarSystem.GREGORIAN month produces a NepaliMonthCalendar holding Gregorian numbers. Convert the month to Bikram Sambat first when that matters.