Ability to download artifacts from all runs in a folder

This should include parallelisation, so that multiple artifacts are downloaded simultaneously, in order to get the best performance.

Could extend `get_artifacts_as_files`, e.g.
```
client.get_artifacts_as_files(run=run_name, path='/tmp', category='output')
```
or:
```
client.get_artifacts_as_files(folder=run_name, path='/tmp', category='output')
```