Throw error when trying to sympify `pystencils.Field` (e.g. using it in an...

Throw error when trying to sympify pystencils.Field (e.g. using it in an Assignment without indexing)

This is a typical error when using pystencils: you forget the index and use a field directly in an Assignment.

Edit: apparently, this error is only triggered on recent versions of Sympy that can sympify using __sympy__ (not on CI).