Adding new property method `dr` and `r`.
Adding new property method `dr` and `r`.

`dr` populates array of all Atom.dr vectors and
`r` populates array of all Atom.r vectors and replaces
the formerly defined property method `coords`.

The `coords` property is now an alias for the `r` property.

scikit-nano/scikit-nano,androomerrill/scikit-nano,scikit-nano/scikit-nano,androomerrill/scikit-nano