Improve testing for addfield.f

[edwardhartnett]

There are a lot of conditions in this code that are not being tested.

[edwardhartnett]

With recent merges much of the easy testing has been done, and remaining testing involves some grib complexities.

1. bitmaps - some GRIB messages have them, but I have no examples to work with (yet).
2. different types of compression, currently only simpack() is tested.

Of these, 2 looks easiest to tackle. Getting JPEG and PNG compression working in tests would be very helpful. This would mean changing the DRS template array (idrstmpl in the parameter list of addfield()). By setting the DRS template to the appropriate size and values, different compression types can be specified.