added natrual abundance to MoessbauerIsotope class

docs/source/news/news.rst

@@ -68,6 +68,8 @@ API changes:
    It enables to use other than least-square prolems, like log-likelihood estimators.
    It does not work with the ceres solver modules, which always solve least-square problems.
 
+#. :attr:`natural_abundance` attribute added to :class:`MoessbauerIsotope`.
+
 
 changes: