Support ad hoc celebration date overrides

[igneus]

fix #80

Church authorities occasionally make an ad hoc change to the calendar, moving a celebration, just for the given year, to a more convenient date beyond the usual calendar rules. This PR proposes one possible way of addressing this.

Shortcomings:

• as it operates solely on the level of celebration dates, but doesn't otherwise affect the logic of celebration precedence, we still aren't able to compute calendar exactly as ruled in the recent CDW response concerning 2022, since Corpus Christi and Birth of St. John Baptist have exactly the same rank and thus the solemnity receiving first Vespers according to the response doesn't receive them according to calendarium-romanum
• it is able to move singular celebrations only (but AFAIK ad hoc moving around has so far never affected e.g. a group of optional memorials on the same date, but always a higher-ranking important celebration; should it nevertheless be needed in future, the logic can be extended)

Comments and and alternative proposals welcome.