Adds support for --update-data downloading updates to --datadir. This is to support environments where pangolin does not have permission to write to its own install directories (such as HPCs or when pangolin is run from a Singularity container).
It also moves the code that looks for pangoLEARN, constellations and pango_designation files up in the file so that this searching is all done before any calls to update() etc.
Includes changes started in PR #282 with fixes to issue #283