Use true bison parser plus the .y files shipped with ngen

[DennisHeimbigner]

You might consider switching to use my 'bison.py' parser skeleton.
(https://github.com/Unidata/bison.py)
If you did, then you could directly use the ncgen.y grammar
to parse full netcdf-4 CDL.

[rockdoc]

@DennisHeimbigner - Hi Dennis. Thanks for letting me know about bison. I'll look into it.