Schema should be a submodule

[mcafaro]

The schema files have currently been copied and pasted into the MatNWB repository. A better approach may be to use a submodule that points to the schema repo. I can think of some pros/cons for both approaches, so this change is not a definite win but something to think about. Submodule pros: avoids copy/paste mistakes, makes the source of the schema files clear (as well as git rev). Cons: confusing for new users cloning the repo (need to use --recursive flag to get submodules)

[lawrence-mbf]

Since we now have users that rely on the source code without using git, we can no longer implement this suggested enhancement without breakage.