tweak imports

nifty5/library/apply_data.py

 def ApplyData(data, var, model_data):
     # TODO This is rather confusing. Delete that eventually.
-    from .. import DiagonalOperator, Constant, sqrt
+    from ..operators.diagonal_operator import DiagonalOperator
+    from ..models.constant import Constant
+    from ..sugar import sqrt
     sqrt_n = DiagonalOperator(sqrt(var))
     data = Constant(model_data.position, data)
     return sqrt_n.inverse(model_data - data)