streamline connection to WRDS in "connect_WRDS.R"

[arndtupb]

new script connect_WRDS.R contains both (i) the request for WRDS credentials and (ii) the actual establishment of the connection to WRDS.

to (i): section on WRDS credentials will

• check for existence of config.csv, if missing will
• check for RStudio environment (e.g. when script is individually executed in RStudio to pull data) and uses rstudioapi::askForPassword for pop-ups to enter wrds credentials, if RStudio is not running
• stop with customized error message

to (ii): extracted & excluded from "pull_wrds_data.R" to keep the script's focus on the download of data (what is pulled, not how). wrds_user and wrds_pwd should be removed, once connection is established.

connect_WRDS.R makes read_config.R obsolet

I also renamed the save_wrds_data function to save_rds_versionctrl because its only applicable to rds-files.

sorry for the many requested changes in 1 pull request. will adapt README once we agree on a final solution.

[arndtupb]

sorry for mixing things in two commits!