close NCDatasets after access in `process_snowmip.jl`

[juliasloan25]

This file accesses two datasets, met_data and snow_data, but never closes them. We just need to add two lines after we're done using them:

close(met_data)
close(snow_data)

[ph-kev]

It would probably be better to use do notation, so it automatically close if there is an error.