[ycexiao]

What problem does this PR address?

Constraint variables are created to hold the common values used by the variables to be constrained.

e.g. lattice can be created to constrain lattice.a, lattice.b, lattice.c to be the same value of lattice.

Create new constraint variables in each solve so recipe get clean setup.

What should the reviewer(s) do?

[ycexiao]

@sbillinge, it's ready for review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (0c7ccf8) to head (32ce3e1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage   99.68%   99.69%           
=======================================
  Files           8        8           
  Lines         634      648   +14     
=======================================
+ Hits          632      646   +14     
  Misses          2        2           

Files with missing lines	Coverage Δ
tests/test_refinement_session.py	100.00% <100.00%> (ø)

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