Faster leverage function

[dmbates]

• at present this only works for a single grouping factor - the P.R. is mainly to show the outline
• needs tests, extension, much cleaning up
• and, of course, needs to be mentioned in NEWS.md

[codecov]

Codecov Report

Merging #553 (c29cc03) into main (ace7f89) will increase coverage by 0.09%.
The diff coverage is 100.00%.

❗ Current head c29cc03 differs from pull request most recent head a80a2a5. Consider uploading reports for the commit a80a2a5 to get more accurate results

@@            Coverage Diff             @@
##             main     #553      +/-   ##
==========================================
+ Coverage   96.05%   96.14%   +0.09%     
==========================================
  Files          27       27              
  Lines        2456     2488      +32     
==========================================
+ Hits         2359     2392      +33     
+ Misses         97       96       -1     

Impacted Files	Coverage Δ
src/linearmixedmodel.jl	97.84% <100.00%> (+0.13%)	⬆️
src/Xymat.jl	90.00% <0.00%> (+2.50%)	⬆️

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[palday]

This does make the power of the 3 block set up much more obvious