Rename SmcKApproxCoalescent to SMCK

[jeromekelleher]

Closes #2449

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.22%. Comparing base (12b3951) to head (c288407).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
msprime/_msprimemodule.c	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2450   +/-   ##
=======================================
  Coverage   91.22%   91.22%           
=======================================
  Files          20       20           
  Lines       11847    11847           
  Branches     2300     2300           
=======================================
  Hits        10807    10807           
  Misses        569      569           
  Partials      471      471           

Flag	Coverage Δ
C	91.22% <87.50%> (ø)
python	98.63% <100.00%> (ø)

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