Add code to download SLCs from JPL

[julober]

I wrote a couple of functions to generate the UAVSAR SLC filenames (from asf search results) and download them from the JPL site, which I thought might be useful to add to this repository. There are a couple of things that I couldn't find in the ASF metadata (what I call tag and the release folder). For the release folder, the code just brute force searches for a valid link, and for the tag, I let the user determine what tags to look for. I tested them in a notebook called slc_download_test.ipynb. Let me know if you have any feedback!

[julober]

After testing the code for myself, I determined that I needed to search both tags (BC/BU) in a similar way as the release folder, so I made that change.

[ZachHoppinen]

This all looks good to me, feel free to merge.

For versioning, I agree it is not the prettiest, but I do not think there is a cleaner option given that ASF's versioning and the SLC stack versioning on the UAVSAR site are independent. The one thing that might help later is resolving the release folder and tag inside get_uavsar_slcs instead of at download time. It already makes a network call to ASF, so one extra probe per flight line is cheap, and it would let you return real URLs rather than the [BC/BU] placeholder and surface a bad version where the user typed it.