Create definition files for NCEP local codes

[weech]

NCEP commonly shares GRIB files that include local codes. As this library uses ecCodes, an ECMWF product, it has an out of date version of that code table. However, ecCodes is designed so that anyone can create definition files for local codes and include them transparently. It would be great if we had up-to-date definition files for NCEP files since they are used so often (see #4 for example for where this causes confusion). This web page describes the syntax.

[weech]

I added a directory to hold the local definitions. Now it's just a matter of adding the new products. There shouldn't be too many, but there are a lot of tables to go through to find all the new ones.