Clarified the import in simmetrica.__init__

In Python 2, this means "from simmetrica.simmetrica import Simmetrica"
but in Python 3 it means "from simmetrica import Simmetrica". To make
this consistent in both versions, import the absolute_import and use
the dot notation to clear import the package relative to the init.

To see the issue, if you do "import simmetrica" in Python 2.x it will error.