Adding 3D-VAR wind retrieval notebook

This is the notebook I do most of my wind retrieval testing in, which as of now
consists of the following steps:

1. Reading multiple radar grids.
2. Quality controlling said grids.
3. Plotting the results of the quality control.
4. Looking at echo base and top heights (optional).
5. Looking at column types, e.g. well-defined, top-defined, etc. (optional)
6. Computing fall speeds and plotting the results (optional).
7. Reading an ARM merged sounding product to be (potentially) used.
8. Running the 3D-VAR wind retrieval.
9. Checking the quality of the results, including plotting.
10. Computing some statistics about the retrieval.
Merge remote-tracking branch 'origin/master'

I believe I had to do a merge instead of a rebase because my local master branch diverged from origin/master...