save files to `tempdir()` by default

[bschilder]

Whenever you have a default location to store files, it's a good idea to make that default a tempdir(). This is requires by CRAN/Bioc checks, but also helps you avoid accidentally including temp files in your package build.

[bschilder]

load_phenotype_to_genes <- function(pheno_to_genes_txt_file = "data/phenotype_to_genes.txt") {
...

to

load_phenotype_to_genes <- function(pheno_to_genes_txt_file =
                                        file.path(tempdir(),
                                                  "data/phenotype_to_genes.txt")
                                    ) {
...