Improve gak

[YannCabanes]

This pull request improves gak in which an overflow can occur when used on long almost constant time series.
This pull request is related to the issue #510 and the problem is detailed here: #510 (comment).
The current gak function always works for time series of length sz <= 204 and can fail for time series of length sz >= 205.
This pull request proposes a solution for gak to always work for time series of length sz <= 405 when the data type of the inputs is float64.