Add named function kernels for sliding windows

[lmcinnes]

For now just a couple of simple test kernels; one for count time series anomaly detection, and another to work with time interval time series anomaly detection, which is similar. Enough basics to test out that the named function code-path works.

[codecov-commenter]

Codecov Report

Merging #86 (78e002c) into master (e7048c0) will decrease coverage by 0.20%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   91.29%   91.09%   -0.21%     
==========================================
  Files          24       24              
  Lines        3861     3975     +114     
==========================================
+ Hits         3525     3621      +96     
- Misses        336      354      +18     

Impacted Files	Coverage Δ
vectorizers/_window_kernels.py	83.94% <73.80%> (-4.61%)	⬇️
vectorizers/transformers/sliding_windows.py	93.00% <88.60%> (-3.59%)	⬇️
vectorizers/tests/test_transformers.py	100.00% <100.00%> (ø)
vectorizers/transformers/__init__.py	100.00% <100.00%> (ø)

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[jc-healy]

Look solid.