estimate_shift1D raised an exception because of numpy.clip function

Dear All, 

I found that "estimate_shift1D" is raising an exception when the kwarg "max_shift" was used. The exception is related with the np.clip function parameters "a_min" and "a_max", which could not be used by name. I guess the solution is not to use these parameter names and pass the values to the np.clip function directly.

I've implemented this solution in this pull request.

Best wishes,
Alberto.