add tensor.where as an alias for tensor.switch to support the same name as numpy.

doc/library/tensor/basic.txt

@@ -986,6 +986,11 @@ Condition
       x,y = T.dmatrices('x','y')
       z = T.switch(T.lt(a,b), x, y)
 
+.. function:: where(cond, ift, iff)
+
+   Alias for `switch`. where is the numpy name.
+
+
 .. function:: clip(x, min, max)
 
     Return a variable representing x, but with all elements greater than

theano/tensor/basic.py

@@ -2605,11 +2605,11 @@ def isinf(a):
 # Condition
 ##########################
 
-@_scal_elemwise
+@_scal_elemwise_with_nfunc('where', 3, 1)
 def switch(cond, ift, iff):
     """if cond then ift else iff"""
 
-
+where = switch
 ##########################
 # Bit-wise
 ##########################