Skipping n-train years that are adjacent to a test year

[BSchilperoort]

At some point, it would also be nice to have the functionality to skip n-train years that are adjacent to a test year. Highly autocorrelated timeseries may lead to information leakage from one year to the next. To ensure this effect is reduced/negated, you can remove the years adjacent to test years from the training datesets.

See gap_prior and gap_after arguments in legacy code:
https://github.com/AI4S2S/proto/blob/77734930a40b8aaefcf1e390efe0e3ac93b40858/RGCPD/class_RGCPD.py#L312-L314 (edited)

Originally posted by @semvijverberg in AI4S2S/s2spy#52 (comment)