Global constraints (Sakoe-Chiba/Itakura) for subsequence search

Is your feature request related to a problem? Please describe.
The method dtw_path accepts one of two optional global constraints (preventing "pathological" matches). However, dtw_subsequence_path doesn't (because the compute mask function assumes two series of equal length).

Describe the solution you'd like
Adding this constraint would be really useful. There is a precedent, the team at UCR has an implementation of DTW subsequence search with a warp window as a fraction of the series length (the source is in C, I think the cost matrix is here). I could try to have a go at this but it might take me a while.

Thanks for all your work on this very useful library!

Start by reading the dtw_path and dtw_subsequence_path entry points, then inspect the compute mask function to understand its equal-length assumption. Compare the requested behavior with the linked UCR implementation and its cost-matrix logic. The work is done when subsequence search accepts the global constraints and constrained paths avoid pathological matches.