Remove timezone support from sas_numeric_to_date()

[billdenney]

While working on #582, there was an issue with timezone tests in sas_numeric_to_date().

Looking at https://communities.sas.com/t5/Ask-the-Expert/Tips-and-Tricks-for-Working-with-Dates-and-Times-in-SAS-Q-amp-A/ta-p/813974, it appears that SAS likely (but not definitively) doesn't support storing time zones and it likely stores the timezone as the current time zone. The documentation that I can find is not clear.

Therefore, I plan to remove timezone support from the function and always use UTC timezones. That seems like the least-bad choice because a SAS file sent to a computer in a timezone that has different daylight saving time method would likely get different results, based on my reading.

[billdenney]

The implementation in #584 doesn't remove timezone support but gives a warning when it's used with any timezone other than UTC.

[sfirke]

The implementation in #584 doesn't remove timezone support but gives a warning when it's used with any timezone other than UTC.

I'm reviewing #584 and want to make sure I understand:

1. is the idea that the input tz value is ignored and UTC used for the conversion, with the only use of the existing tz argument being to warn users who have legacy code using that argument?

2. If that's right, does that mean this will be a breaking change for those users because the same input will now result in a warning and a different result?

[billdenney]

1. tz is not ignored. Now UTC is used by default but the user can specify a different tz value.
2. It is a breaking change that tz used to be "" by default and now it's "UTC". And, yes, there is a warning which is technically a breaking change (but the warning doesn't change the calculation results).