Add OpenMP directives

[EricEngle-NOAA]

There are several places where OpenMP directives can be added for faster performance -- mainly in the packing and unpacking functions were data are scaled (unscaled) and packed (unpacked).

Introducing this would also mean changes to the CMake configuration files so the OpenMP library can be linked.

[edwardhartnett]

@EricEngle-NOAA is this still a work in progress? What went wrong with your PR?

[edwardhartnett]

Any progress on this? SHould I bump to next release?

[EricEngle-NOAA]

No progress. Please table for the next release.

[edwardhartnett]

We are now working more closely with the wgrib2 project, which (I think) has openmp everywhere. So it might be worth comparing codes. We can ask @webisu for lessons learned, or, even better, just cut and paste his already-working code! ;-)

I have bumped this to the next release.