Add Arg::Convert() method and Arg class constructor that indexes an item.
The Arg::Convert() method is a wrapper around ArgCvt(). The Extra constructor makes it easy to index into an SDS array and get an Arg as a result. Added .gitignore file with a list of all the files to be ignore as seen on an aaolxp build. Bump the DRAMA version to 3_99 (dmakefile)