Changing `Atom` class constructor logic.
Changing `Atom` class constructor logic.

Updating class constructor to use either the element
or mass kwarg to set the `Atom` element value.

Also adding methods for `rotate` and `translate` which
assert that the super class object does not have
said attribute.

Also adding `fmtstr` attribute and setter/getter methods.

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