Add symphony-core to the symphony-matlab repo as a submodule

[cafarm]

When a release of Symphony is made, the symphony-matlab repo is tagged but the symphony-core repo is not. Because of this, there's not a great way to go back and look at the state of the core source code at particular release versions.

If we add the core repo as a submodule to the matlab module, the matlab repo tag will allow us to preserve the revision of the core repo used to make that release. It will also make it easier to automate more of the release process, perhaps with a "release.m" or "deploy.m" script.