SW-415: do not overwrite file, but append to it if existing. Issue warning to fill in the new credentials

SW-415: automatically create dbcredentials file with sane defaults when no credentials are found. added tests.

Task #10168: Raise exception if requesting non-existing credentials, resulting in cleaner code, and added test.

Task #8899: Credentials now have their own class, and a specific function to convert to str and to pg.connect options

Task #8899: Propagate username, password, port to pg.connect. Also, return dbcreds["port"] as an int