6. ) assertions should be added to all other functions where `ref_path`

[munoztd0]

6. Notice the assertions for ref_path that I added at the beginning of a_freq_j(). Similar (or the same) assertions should be added to all other functions where ref_path is used. However, in the context of this PR, please limit yourself to the functions that use h_get_cur_trt_grp().

   Perhaps it would be useful to introduce a small assertion helper, such as assert_col_path(x, ..., min.len, len), since we often need to perform two separate checks when validating the column-split path.

Originally posted by @wwojciech in #423 (review)