Equal constraints

[matscalia]

Adds the Equal and EqualRadius constraints metioned in #1979.
While I was writing the tests and adding small perturbations to test the solver I noticed that the optimizer would not converge for some Angle constraints. With the wrap around in angle_cost, the delta is always minimal and the solver converges. No tests broke and I couldn't think of any case in which this modification would actually change the already established behavior but I would ask for a double, triple check.

[matscalia]

nevermind, I definitely didn't understand how the angle constraint worked, the wrap around was a mistake. I'll fix it as soon as possible, hopefully tomorrow.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.95%. Comparing base (69e9514) to head (f27bf47).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1983      +/-   ##
==========================================
- Coverage   95.96%   95.95%   -0.01%     
==========================================
  Files          29       29              
  Lines        7857     7896      +39     
  Branches     1174     1185      +11     
==========================================
+ Hits         7540     7577      +37     
- Misses        183      184       +1     
- Partials      134      135       +1     

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