better error message (and partial code clarification)
add Tri op, tri/tril/triu functions
add triangle tests
add Nonzero Op, function, tensor method
add nonzero test
move Nonzero docstring from the function to the class
add 1D test for nonzero
add flatnonzero and tests
add nonzero_values and test, also update nonzero docstring to reflect new indexing behavior
Implemented c_code for AdvancedSubtensor1
fix a NotScalarConstantError issue
Make check_preallocated_output work on fct outputs

In DebugMode, do not use the no_recycling list when calling the CLinker
to build individual thunks, so that output memory can always be re-used,
even for outputs of the actual function.

This enables check_preallocated_output to work on Ops that compute
the function's outputs.

This should close gh-1174.
Add comment after code review
change Op to return matrix; change function to return either tuple or matrix
fix typo in docstring
missing if/else for return_matrix
guarantee zeros in tests
Add test for when b is not constant
avoid vstack of 0-length arrays; cast result as int
make sure numpy.nonzero returns tuples with length > 0 before numpy.vstack