Reduce memory footprint of optimization logic

src/cate/core.py

@@ -56,6 +56,8 @@ def _pairwise(iterable):
 class _OptimizerState:
     """Internal state of the optimization logic to compute crosslevels."""
 
+    __slots__ = ('position', 'crossings', )
+
     def __init__(self, position, crossings):
         self.position = position  # current position of strands
         self.crossings = crossings  # remaining crossings