Skip site positioning for simple cases.

[jeromekelleher]

Closes #73.

[codecov-io]

Codecov Report

Merging #74 into master will increase coverage by 0.02%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
+ Coverage   92.62%   92.65%   +0.02%     
==========================================
  Files          10       10              
  Lines        2415     2422       +7     
  Branches      412      414       +2     
==========================================
+ Hits         2237     2244       +7     
  Misses        149      149              
  Partials       29       29

Impacted Files	Coverage Δ
tsinfer/inference.py	98.34% <90.9%> (+0.02%)	⬆️

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