Define a subset of the ToxCast database for relevance to toxEval analyses.
Subsetting is done based upon methods defined by Blackwell et al., 2017 (
tools:::Rd_expr_doi("10.1021/acs.est.7b01613")).
Specifically, this function removes endPoints that are ATG sources with
signal loss, and NVS with signal gain (basically: some assay/signal combinations
are removed because they target non-specific endpoints). Also, this function adds additional
categories to intended_target_family and intended_target_family_sub as
described in the paper linked above.