Unknown country code in NHIA algorithm about algorithms

[depryf]

"SWK" is the old SEER code for "Slovakia", but it was changed to "SVK" a few years ago. I am not sure why this algorithm was not updated (especially since I see other country codes that were updated).

I am going to investigate a little bit more before fixing this.

[depryf]

This library is supposed to implement the same logic as the corresponding SAS program provided by NAACCR (https://www.naaccr.org/analysis-and-data-improvement-tools/#NHAPIIA). It turns out that program is also missing the new "SVK" code. That is going to be fixed and a new version is going to be uploaded on the NAACCR website; when that happens, I will add the new code to this library as well.

[depryf]

The SAS code has not been fixed yet, but it should be soon. Since this library is going to be released soon, I added the missing "SVK" code to that list.