Add CBA_fnc_weekDay

[jonpas]

When merged this pull request will:

• Add a function that calculates date's day of the week using a modified Rata Die method with fractional years.
• Add unit tests for the function.

[commy2]

CBA_fnc_weekDay

https://en.wikipedia.org/wiki/ISO_week_date

Should sunday be 0 or 7?
Is -1 a reasonable fallback?
Could the function or a variant thereof return strings, e.g. "MON"? Perhaps even (translated) stringtable entry?

[jonpas]

Whichever.
Yes.
Sure.

[jonpas]

For reference, -1 will only return on invalid input. If we switch to 1-7 for days, invalid could also be 0.

[TheMagnetar]

I would prefer -1 to be invalid. A return of 0 can be confusing since it can be interpreted still as valid. A -1 is "visually" always invalid for days of the week in all conventions

[PabstMirror]

[[2101,1,1]] call CBA_fnc_weekDay; = 0
but https://www.timeanddate.com/calendar/?year=2101 says it should be a saturday
I think it's because 2100 isn't a leap year (even though it's divisible by 4)

hopefully arma 4 is out by then, so this shouldn't really be an issue 😄 (max mission date in arma is 2050)