unified way of handling downloaded models/files

[rbroc]

Need to settle on a way to handle non pip-installable modules and files. At the moment, this is done with with ad-hoc scripts and MissingDependencyError messages for nltk and yamnet. It might be worth thinking about a standard general way in which these cases are handled. Ideally, for a missing module, the user should be able to run something like pliers install module_name --dir=/path/to/put/files from the command line, where the storage location is optional (and would default to what we're currently using, i.e. pliers_data in the user's home directory).