Follow-up from "Add medium type, as new property (air, water, rock...)"

The following discussion from !265 (merged) should be addressed:

• @msackel started a discussion: (+5 comments)

  I'm not entire sure why the medium type is a function of the position. I thought in an environment itself the nuclear compositions and the media constants do not change or is parameterized by a model?

      virtual Medium medium() const = 0;