Added method to compute dtw distance using custom metric without the path

[sw241395]

Added in a method too compute the DTW distance between 2 time series using a custom metric but not generate the path.

Overall computing DTW can be quite a slow process, so i though if you was only interested in the distance then you could help speed up the code buy not creating the path. I have tried to keep it consistent with the rest of the code but happy to made stylistic changes if needed.

Overall still new to contributing to open source so happy to make any changes and feedback is always appreciated.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 :tada:

Comparison is base (d8c1930) 94.57% compared to head (08e068e) 94.60%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #452      +/-   ##
==========================================
+ Coverage   94.57%   94.60%   +0.02%     
==========================================
  Files          62       62              
  Lines        4738     4762      +24     
==========================================
+ Hits         4481     4505      +24     
  Misses        257      257              

Impacted Files	Coverage Δ
tslearn/metrics/__init__.py	100.00% <ø> (ø)
tslearn/metrics/dtw_variants.py	97.19% <100.00%> (+0.08%)	:arrow_up:
tslearn/tests/test_metrics.py	100.00% <100.00%> (ø)

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[rtavenar]

Hi @sw241395

Thanks for this suggestion. You are perfectly right that this should be offered as a direct function to the end-user.

Here is my quick feedback:

1. This looks great (almost ready for integration)
2. I would rather call the function dtw_from_metric

[sw241395]

Hello @rtavenar

I have made the change. Hope it all looks ok. Happy to make other changes if needed.