more tests and documentation for tocgrib2

[edwardhartnett]

We have some tests (see #324) but need more.

Also can we have more documentation about this utility? What applications use it?

[AlexanderRichert-NOAA]

Ops models that appear to use tocgrib2: ecmwf etss gefs gfs glmp glwu hiresw href hrrr lmp nwps rap rtma rtofs seaice_analysis sref urma wsp

[edwardhartnett]

Can you update the readme and/or the tocgrib2 documentation? Also what about tocgrib2super?

[AlexanderRichert-NOAA]

Sure. As for tocgrib2super: aqm, glmp, rap