Merge pull request #1389 from nouiz/sigmoid

Sigmoid
better comment.
remove import not used.
Allow other file to reuse tensor/tests/test_basic.py:MakeTester
Add ultra_fast_sigmoid.
Add SigmoidTester and SoftplusTester
Allow to pass rtol and atol to TensorType.values_eq_approx
Added doc about ultra_fast_sigmoid.