Create concise requirement and env files

[andrewtavis]

This issue is for creating concise versions of requirements.txt and environment.yml for causeinfer. It would be great if these files were created by hand with specific version numbers or generated in a way so that sub-dependencies don't always need to be updated.

As of now both files are being created with the following commands in the package's conda virtual environment:

pip list --format=freeze > requirements.txt  
conda env export --no-builds | grep -v "^prefix: " > environment.yml

causeinfer and other obviously unneeded packages are then removed from these files before being uploaded.

Any insights or help would be much appreciated!