Configuration for model locations

[buddha314]

Allows for abstract model locations away from AWS. This file sets AWS as model source

https://github.com/allenai/scispacy/blob/main/scispacy/candidate_generation.py

For instance https://github.com/allenai/scispacy/blob/main/scispacy/candidate_generation.py#L43

Allow these to be set by environment or configuration files.

[dakinggg]

Makes perfect sense @buddha314, I'd be happy to accept a PR doing this!

[buddha314]

Initial thinking is to take the root url as an environment variable with the AWS location as the default.

[dakinggg]

Makes sense, I might consider making a new LocalLinkerPaths that reads from environment variables.