Make optional sampler args keyword-only

[arcondello]

This helps avoid confusing results when switching between samplers.

This is a backwards compatibility break, but a minor one so I think it's safe.

[codecov-commenter]

Codecov Report

Merging #1316 (fa6e6d6) into main (d5a3059) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main    #1316   +/-   ##
=======================================
  Coverage   91.69%   91.69%           
=======================================
  Files          94       94           
  Lines       10126    10126           
=======================================
  Hits         9285     9285           
  Misses        841      841           

Impacted Files	Coverage Δ
dimod/reference/samplers/exact_solver.py	99.02% <100.00%> (ø)
dimod/reference/samplers/identity_sampler.py	88.88% <100.00%> (ø)
dimod/reference/samplers/random_sampler.py	100.00% <100.00%> (ø)
dimod/reference/samplers/simulated_annealing.py	100.00% <100.00%> (ø)

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[randomir]

LGTM!