Make sure that Custom Expression is only evaluated on coupling boundary

[BenjaminRodenberg]

https://github.com/precice/fenics-adapter/blob/a2b5b5d0d6692fcd3e6e32ef618c4c2ec8cc5128/fenicsadapter/fenicsadapter.py#L62-L63

This function returns invalid values if evaluated somewhere outside of the coupling boundary. We should put an assertion or similar here.