Enforce Isomorphic schema contraints

Hi there,

I have some time to look again at project-m36 and tried isomorphic schemas again launching off what I had tried in https://github.com/agentm/project-m36/issues/223#issuecomment-436585971 from the example.

I went a bit deeper and looked at the [code for IsoRestrict](https://github.com/agentm/project-m36/blob/master/src/lib/ProjectM36/IsomorphicSchema.hs#L134). So the predicate for IsoRestrict actually doesn't do anything (yet?).

I also saw your issue https://github.com/agentm/project-m36/issues/55 and it seems that at the time, `relExprMorph` didn't return an `Either`. Now that it does, should the isomorphic schema constraints be enforced here?