Add explicit local models

nifty4/models/__init__.py

 from .constant import Constant
-from .local_nonlinearity import LocalModel
+from .local_nonlinearity import (LocalModel, PointwiseExponential,
+                                 PointwisePositiveTanh, PointwiseTanh)
 from .model import LinearModel, Model
 from .variable import Variable
 
 __all__ = ['Model', 'Constant', 'LocalModel', 'Variable',
-           'LinearModel']
+           'LinearModel', 'PointwiseTanh', 'PointwisePositiveTanh',
+           'PointwiseExponential']

nifty4/models/local_nonlinearity.py

+from nifty4.library.nonlinearities import Exponential, PositiveTanh, Tanh
 from nifty4.sugar import makeOp
 
 from .model import Model
@@ -18,3 +19,15 @@ class LocalModel(Model):
 
     def at(self, position):
         return self.__class__(self._inp.at(position), self._nonlinearity)
+
+
+def PointwiseExponential(inp):
+    return LocalModel(inp, Exponential())
+
+
+def PointwiseTanh(inp):
+    return LocalModel(inp, Tanh())
+
+
+def PointwisePositiveTanh(inp):
+    return LocalModel(inp, PositiveTanh())