Add SWE dataset

[lekoenig]

Let's make a new issue to add a SWE dataset. Not a current need. I'd like to see how models are performing in winter with the current met dataset before adding a new dataset.

Originally posted by @jds485 in #184 (comment)

[lekoenig]

Here are some notes about what the regional drought project is using, from Scott Hamshaw:

We’re using a couple data products on the drought project related to snow.
For the 1981 – 2020 period, we are using the NSIDC SWE data (4km, daily, CONUS) https://nsidc.org/data/nsidc-0719/versions/1

For the 2000 – 2020 period, we’ve also brought in MODIS derived snow covered area – we haven’t evaluated this one much yet though

We also have a SWE variable modeled from the WLDAS western land surface model, but that wouldn’t be available across CONUS

[jds485]

Andy Bock suggested SNODAS, which is daily 1 km resolution model estimates available since 2003