Improve enum (#173)

* fixes math.ceil with explicit cast to double before ceiling

* makes error codes more human readable

Author: derNarr

pyndl/__init__.py

@@ -17,7 +17,7 @@
 __author__ = ('Konstantin Sering, Marc Weitz, '
               'David-Elias Künstle, Lennard Schneider')
 __author_email__ = '[email protected]'
-__version__ = '0.6.3'
+__version__ = '0.6.4'
 __license__ = 'MIT'
 __description__ = ('Naive discriminative learning implements learning and '
                    'classification models based on the Rescorla-Wagner '