Apply column mapping to twas pipeline

[rl3328]

1. When loading GWAS summary statistics, specifying a column_file_path (YAML format) enables the use of load_rss_data(). This function standardizes column names and can flexibly generate missing "z" or "beta" columns using the col_to_flip option. Whereas when column_file_path not provided, it uses the default simple and computationally efficient tabix_region().
2. Add --comment-string , which comes with load_rss_data()