Use submodule to pull in llms-with-matlab

[ccreutzi]

To better track development in llms-with-matlab, it seems like a good idea to use the standard mechanism of git submodules instead of maintaining a local copy that does not track which version of the code it actually includes.

[nothans]

Note for future Hans using GitHub Desktop client.

If you get a fatal error for submodules, you need to clone from the branch from the command line into a folder and then open an existing repository.

git clone -b {branch_name} {repo_url}
git submodule init
git submodule update