Add an mdt update CLI command

[jrlegrand]

Proposal

We currently have mdt init as the command to download the databases and do some data crunching. If you want to update the databases, currently you have to delete the /data folder and re-run mdt init. Would prefer to have an mdt update command that does that for you.

Rationale

More user friendly - periodically would want to make sure we have the most recent RxNorm data and possibly MEPS data - though I think we are still hard coded to 2018 data.