Add a Calmarendian date-time Data Structure

[natashamoorfield]

Not a full-fat CalmarendianDateTime class, per se, but a data structure containing all the elements needed to completely define a Calmarendian date-time object, but no more. This will be the equivalent of the time.struct_time object in the Python Standard Library but, rather than use a named tuple, we might want to consider a data class or some such.

[natashamoorfield]

What actual purpose does this data structure serve? What uses it? Should we not at least have the ability to (re)instantiate a CalmarendianDate object from an instance of it?