The calendar enumerator demo now includes inferior and superior
conjunctions of the inner planets (Mercury and Venus), along
with the conjunctions and oppositions of the outer planets
(Mars ... Pluto).
Once a collator has been created, and a caller starts
enumerating events, it does not make sense to be able to add
another enumerator to the collator. So I removed EventCollator.Append().
It was just opening up the possibility of bugs for no good reason.
Client code should decide up front what kind of events it wants
to enumerate and provide a complete list of enumerators.
Then it may use FindFirst/FindNext as many times as it wants
and everything will just work.
Added MoonQuarterEnumerator, which finds new moon, first quarter,
full moon, and third quarter events.
Changed the calendar start date to May 2021, so it is
more relevant to the time I'm testing it.
This is an example of how multiple enumerators can be combined
into an EventCollator. The collator does the minimum amount
of work to keep searching for one event at a time, while always
emitting them in chronological order.
