It is generally useful for dummy atoms to have radius

In order to be rendered, and viewed, giving dummy atoms a radius is
useful. Making it around half that of a hydrogen makes it clear they are
not real, they already had a color assigned and a zero mass certainly
seems correct.

Change-Id: Ib9175e7ef7000e203414c9bd7390e2d1ce213c32
Merge topic 'dummy-atom-radius' into master

c9c0e81e It is generally useful for dummy atoms to have radius