Get directory to store back end files like kingdom summaries etc
cachedir(non_temp_cache = "~/.biomartr_cache_dir.rds")
"~/.biomartr_cache_dir.rds",
reads the rds file, and returns the path for local cache, if not existing, use tempdir().
Other cachedir:
cachedir_set()
cachedir()
#> [1] "/var/folders/yn/mgwl8_b56hz4v2c2vlfxj07w00076j/T//Rtmp3PdEM5"