create "extra" subdirectory at the top level, and move user-contributed code that we will not support into it

[edwardhartnett]

We have features like the mysql compatibility, which were contributed by users but are of no interest to NOAA.

We will not write testing or documentation for these codes.

Instead, let's move them to an "extra" directory at the top level. They extra directory can be optionally built. When built, it's CMakeLists.txt builds the code in the extra directory into a library that the rest of wgrib2 links to.

@webisu , other than the mysql code, what other features are extra?

[AlysonStahl-NOAA]

I'm not sure why, but I just caught that there are two projects titled wgrib2-3.5.0. The duplicate one contained this issue and one other closed issue.

I just moved this and the other issue to the correct project.

[edwardhartnett]

Probably my fault. Delete the unwanted project...