Update nonstationary

[timsainb]

Updates nonstationary version of algorithm to be more uniform with stationary version, by using moving mean + std to do noise thresholding.

This will create a small breaking change for current pipelines, since the parameters for the nonstationry have now switched.
mainly, n_std_thresh_stationary is now n_std_thresh, and shared across stationary and nonstationary.

[nuniz]

Hi Tim
If we're planning non-backwards compatible changes, we should also think about other updates for version 4, like adding the TensorFlow implementation or a refactor I've been thinking about.

How about we create a branch for version 4 and discuss this later?