Add a .gitignore file to keep repo clean

[rofrano]

Need to the standard GitHub Python .gitignore file with the following additions to keep the repo clean:

# Mac OS Metadata
.DS_Store

# Generated files from TCA
model_objects/
ontologies/
*.db

[rofrano]

I didn't want to add the .gitignore file in the previous pull request #90 because I needed to have GitHub delete the files we were about to tell it to ignore in the future. I didn't see a need for a pull request just to add the .gitignore file as a second step so I pushed directly to main.