add_strategic_rivalries() will include some other information derived from the rivalry data that the
user may not want (e.g. start year of the rivalry). Feel free to select those out after the fact. Function includes
an on-the-fly adjustment for Austria for rivalry #79. In this case, the Austria-Serbia rivalry continues for two years
after Austria-Hungary (ccode: 300) became Austria (ccode: 305).
The across_types argument is optional and observed for only state-year calls. It defaults to 1. At the default,
the function looks into the rivalry data (in td_rivalries) and focuses on the type1 column. If, say, a state has
an ongoing rivalry and it is primarily spatial, it codes that as a spatial rivalry. Assume you input across_types = 2, the
function then looks across both the type1 and type2 columns to see if there is a spatial component to the rivalry as
either its primary or secondary dimension. If so, it codes that as a 1. across_types must be 1, 2, or 3.