added check for array in diffusion constant errors

Fix for discord issue

The issue probably started a while back when the typing checks were implemented, causing some changes in the normalizers. In principle this specific quantity could be made into just a float I think, but I would like to wait till we tackle the refactoring of this schema to decide.

I added the if/else for compatibility with existing entries and for either structure in the future. Please someone, e.g., @amgo , if this is an ok approach in general, and if js code is ok.