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 https://github.com/tslearn-team/tslearn/issues/510 and the problem is detailed here: https://github.com/tslearn-team/tslearn/issues/510#issuecomment-2008557358. 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.