Add write_corr_coef_obj_to_csv() helper and import utils::write.csv

[baylinux01]

This PR introduces a new exported function write_corr_coef_obj_to_csv() that saves both the correlation matrix and the corresponding p-value matrix (returned by corr_coef()) into a single CSV file, reproducing the common six-block manual workflow:

1. Header
2. Correlation header + matrix
3. Empty line
4. p-value header + matrix

Changes:

-R/write_corr_coef_obj_to_csv.R: new file
-NAMESPACE: added importFrom(utils, write.csv)
-man/write_corr_coef_obj_to_csv.Rd: auto-generated documentation

R CMD check:

-0 errors / 0 warnings / 0 notes related to this PR
-Pre-existing ggplot2 linewidth issue in mgidi examples remains (unrelated)

Usage example:

library(metan)
cc <- corr_coef(df, c(v1, v2, v3))
write_corr_coef_obj_to_csv(cc, "my_corr")