Rename lagmat to sliding_window

[klane]

• Rename lagmat to sliding_window
• Rename file with sliding window tests

[codecov]

Codecov Report

Merging #115 (26b7ed4) into main (28cd562) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #115   +/-   ##
=======================================
  Coverage   18.48%   18.48%           
=======================================
  Files           7        7           
  Lines         395      395           
=======================================
  Hits           73       73           
  Misses        322      322           

Impacted Files	Coverage Δ
torchts/utils/data.py	39.18% <100.00%> (ø)

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