Streamline checking for MESA_DIR.

Checking for existence of MESA_DIR is now abstracted out to its own private
method, check_for_mesa_dir.
Fix bug in mesascript check and add default command.

Fix a logic bug in check_for_mesascript. Also add new default command which
opens up the defaults file for a star namelist, which is given as an argument.
For example,

>>>> mesa default star_job

will call $EDITOR $MESA_DIR/star/defaults. Assumes the EDITOR variable is set up
to something that can open text files.