export srs_diff_est

[avehtari]

loo_subsample() can sometimes be over-complex. srs_diff_est() is a non-exported internal function used by loo_subsample. srs_diff_est() implements difference estimator Equations 7-9 of Magnusson et al., 2020. It would be useful to export this function and provide user targeted function with documentation.

[jgabry]

Sounds good