Use yaml.safe_load for training scripts

[ernesto-elsaesser]

#142 updated the YAML loading in the download module.

But the training scripts still use yaml.load instead of yaml.safe_load, e.g.:
https://github.com/openvax/mhcflurry/blob/master/mhcflurry/train_allele_specific_models_command.py#L145
https://github.com/openvax/mhcflurry/blob/master/mhcflurry/train_pan_allele_models_command.py#L306

With PyYAML 6.0 (released in 2021) this does not just print a warning, but crashes with an error.
Could you adjust these scripts accordingly?

[timodonnell]

Thanks for noting this - will take a look