Updated JointFCalendar interface & methods

UPDATED:

* calendars field is an Array rather than a tuple of SingleFCalendar’s
* replaced join method with overloaded `+` operator
* type of calendar field for AUD & NZD to JointFCalendar given the
inner constructor method’s restrictions on what JointFCalendar should
be expected
* Updated test cases

NEW:
* `*` method applied to calendars. Will join on good days rather than
on bad as is the case with `+`.