Keep pangolin up to date automatically

[fmaguire]

Making this an issue so I don't forget about it while I think about ways to implement it:

• Add rule that runs pangolin --update before pangolin is run on samples
• Come up with some way to have this run every time, and to rerun the pangolin assignments (and other downstream steps) if pangolin gets updated

[jts]

@rdeborja mentioned to me after the call that we often do our runs on HPC nodes without internet access, so we'll have to work around that for our local builds