init ar1_whitenoise

[JanJereczek]

Implemented:

• ar1_whitenoise function to estimate the AR1 regression coefficient based on alanytical solution of least-square
• test with a known AR1 process
• benchmark to test 0 allocation

[Datseris]

@JanJereczek maybe you should add in this PR a function that actually uses the output of AR1 to detect a transition or something. Like detect significant changes in AR1 or something like that?

[Datseris]

We should have one change_point_detection function that searches for significant differences in the observable over a rolling window. Here the observable is just AR1, but it could also be permutation entropy. Later, we can add a different function early_warning_signal that detects smooth increase of the observable. but for now lets keep it simple with the change_point_function?