Switch finetune and smiles app to use entrypoints

[cthoyt]

This PR does the following:

1. Encompasses the contents of if __name__ == '__main__': ... in both molbert.apps.smiles and molbert.apps.finetune in respective main() functions
2. Introduces python entrypoints in the setup.py so the smiles and finetune scripts get vanity commands in the shell upon pip install. Respectively, they are molbert_smiles and molbert_finetune. This makes it easier to write reproducible scripts that don't rely on the installation to be in a certain place
3. Update the README to reflect the changes in best way to run these apps